*446OPINION
By the Court,
Young, J.:The State charged appellant James Edward Griego (“Griego”) with sixteen counts of sexual assault with a minor under fourteen years of age and five counts of lewdness with a minor. He stood accused of forcing two neighborhood children, John and William M., into various sex acts over a period of time and of fondling Robert C., a third neighborhood child. A jury convicted Griego of twelve counts of sexual assault with a minor under fourteen years of age and three counts of lewdness with a minor. He received consecutive sentences of life with the possibility of parole for each of the sexual assault convictions and ten years for each of the lewdness convictions.
On appeal, Griego argues, inter alia, that: (1) the district court erred by failing to grant his motion for a mistrial following a reference by the prosecutor in front of the jury that Griego was incarcerated; (2) his sentences violate the eighth amendment; (3) the evidence as to count XXI was insufficient to sustain a conviction; (4) Robert C. was incompetent to testify; (5) the district court erred in denying Griego’s motion to sever the count involving Robert C. from the counts involving John and William M.; and (6) the district court erred by denying Griego’s request to have a defense expert in psychiatry or psychology examine John and William M.
MOTION FOR MISTRIAL
During cross-examination, Griego stated that he believed the *447father of two of the children was “out to get him.” The prosecutor replied, “Okay. And that’s why you are in jail; is that correct?” Defense counsel immediately moved for a mistrial. The district court denied the motion, stating that an officer had been escorting Griego in and out of the courtroom and that surely the jury knew he was in custody.
Griego’s assertion of error cannot prevail. The jury knew that Griego was in custody and the oral reference by the prosecutor simply was additional proof of what the jury already knew. Prejudice does not result from the jury hearing proof of what it has already seen. See Leonard v. State, 108 Nev. 79, 824 P.2d 287 (1992). We also note that prior to the prosecution’s reference that Griego was in custody, two of Griego’s own witnesses remarked that he was in jail. We therefore hold that the district court properly denied Griego’s motion for a mistrial.
EIGHTH AMENDMENT
Griego argues that his sentences violate the eighth amendment, asserting that they only serve as retribution.
Griego’s argument is not convincing. In Scott L., A Minor v. State, 104 Nev. 419, 422, 760 P.2d 134, 136 (1988), we stated that “[rjetribution or just deserts as a response to criminal law violation is thought by many jurists and social theorists to be archaic and inappropriate. We disagree.” Id. It is true that Griego received the mandatory sentence for the sexual assault convictions and the maximum sentence for the lewdness convictions. NRS 200.366(2)(c); NRS 201.230(1). However, we will not superimpose our views on lawful sentences pronounced by district court judges, even if we might advocate a more lenient sentence. Sims v. State, 107 Nev. 438, 440, 814 P.2d 63, 64 (1991).
Moreover, a sentence within the statutory limits is not cruel and unusual punishment where the statute itself is constitutional. Lloyd v. State, 94 Nev. 167, 170, 576 P.2d 740, 742 (1978). Griego’s sentence on each count is within the statutory limit and he does not challenge the constitutionality of the sentencing statutes. We therefore find no merit in this argument.
SUFFICIENCY OF THE EVIDENCE
Griego argues that the State presented insufficient evidence upon which to convict him of count XXI, which charged him with lewdness with a minor (Robert C.). Griego argues that inconsistencies in Robert C.’s testimony rendered the evidence insufficient to convict him.
*448The record reveals only two inconsistencies involving Robert C.’s testimony. First, Robert C. was unclear as to when he told his parents that Griego had fondled him; he testified at one point that he told his mother right away and at another point that he told his parents a few days after the incident. However, this court has acknowledged that children are often unable to articulate specific times of events. See Cunningham v. State, 100 Nev. 396, 400, 683 P.2d 500, 502 (1984). Robert C.’s uncertainty about the time of reporting the incident does not render the evidence insufficient to convict Griego on this count.
Second, Robert C. gave conflicting testimony regarding whether Griego had fondled him through the outside or the inside of his clothing. We conclude that this inconsistency did not prejudice Griego, as he was charged with simply fondling Robert C.; the charge did not specify whether the fondling was on the outside or the inside of Robert C.’s clothing.
Griego bolsters his allegation of insufficient evidence by directing our attention to the fact that several young people from his neighborhood testified that he never exhibited any sexual deviancy toward them. It is within the province of the jury to determine the credibility and weight of conflicting testimony and we will not disturb a jury’s findings where substantial evidence supports the verdict. Rice v. State, 108 Nev. 43, 45, 824 P.2d 281, 282 (1992). Here, substantial evidence supported Griego’s conviction on count XXI and we decline to disturb that conviction.
ROBERT C.’S COMPETENCE TO TESTIFY
Griego next argues that Robert C. was not competent to testify. The standard of competence which a child witness must demonstrate is that he has the capacity to receive just impressions and possesses the ability to relate them truthfully. Felix v. State, 109 Nev. 151, 173, 849 P.2d 220, 235 (1993). A trial court’s finding of a child’s competence to testify will not be disturbed absent a clear abuse of discretion. Moore v. State, 105 Nev. 378, 380, 776 P.2d 1235, 1237 (1989).
Griego did not request to conduct a voir dire examination of Robert C. prior to his testifying at trial, nor did he lodge an objection with the trial court regarding Robert C.’s competency. Failure to object at trial results in the issue not being properly preserved for appeal and precludes appellate review. Emmons v. State, 107 Nev. 53, 61, 807 P.2d 718, 723 (1991). We therefore decline to address this issue.
*449 SEVERANCE OF COUNTS
Griego contends that the court should have severed the count involving Robert C. from the other counts charging him with other acts of sexual assault because the charge was not based on the same act or transaction as the other charges, nor was the charge part of a common scheme or plan. We disagree.
Pursuant to NRS 173.115, offenses may be joined. NRS 173.115 provides:
Two or more offenses may be charged in the same indictment or information in a separate count for each offense if the offenses charged, whether felonies or misdemeanors or both, are:
1. Based on the same act or transaction; or
2. Based on two or more acts or transactions connected together or constituting parts of a common scheme or plan.
Joinder in this case was proper. The offenses constituted transactions which constituted part of a common scheme under NRS 173.115 for the following reasons: the count involving Robert C. took place during the same time period as the other counts; the victims were all young boys; the victims were all friends of the Griego children; and Griego allegedly assaulted all three boys in Griego’s home.
Additionally, the evidence was also admissible to show opportunity under NRS 48.045, which provides in relevant part:
2. Evidence of other crimes, wrongs or acts is not admissible to prove the character of a person in order to show that he acted in conformity therewith. It may, however, be admissible for other purposes, such as proof of motive, opportunity, intent, preparation, plan, knowledge, identity, or absence of mistake or accident.
If evidence of one count is admissible in evidence at a separate trial on another charge, the counts need not be severed and may be tried together. Mitchell v. State, 105 Nev. 735, 738, 782 P.2d 1340, 1342 (1989). Here, the district court could have admitted the evidence in the counts involving Robert C. in a separate trial on the counts involving John and William M. Furthermore, because several of the assaults took place in Griego’s home, the district court could have allowed the evidence to show that Griego had an opportunity to assault the children. Thus, Griego’s argument is unpersuasive.
Due to the foregoing, we find no error with regard to count *450XXI (lewdness with a minor, Robert C.) and hereby affirm the conviction on that count only.
ASSIGNMENTS OF ERROR AS TO JOHN AND WILLIAM M.
One assignment of error as to John and William M. is disposi-tive of all the counts involving them. Based on our recent decision in Keeney v. State, 109 Nev. 220, 850 P.2d 311 (1993), we conclude that the district court erred in denying Griego’s request for an independent psychological examination of both John and William M. We stated in Keeney that
it would be error to preclude a defendant from having an alleged child-victim examined by an expert in psychiatry or psychology if: (1) the State has employed such an expert; (2) the victim is not shown by compelling reasons to be in need of protection; (3) evidence of the crime has little or no corroboration beyond the testimony of the victim; and (4) there is a reasonable basis for believing that the victim’s mental or emotional state may have affected his or her veracity.
Keeney, 109 Nev. at 226, 850 P.2d at 315. A review of each of these factors follows.
Whether the State has employed an expert in psychiatry or psychology
The State employed at least two experts in psychology who testified that John and William M. may have been victims of sexual assault. Thus, the first factor suggests that the district court should have permitted Griego to employ his own expert to examine those children.
Victim not shown by compelling reasons to be in need of protection
The record reflects no showing — compelling or otherwise— with regard to whether John and William M. needed protection from further examination by a mental health expert. Further, the district court in this case did not expressly find a compelling reason why Griego’s mental health expert should not examine John and William M. We hold that should a district court conclude that compelling reasons exist to protect a child from examination by a defendant’s mental health expert, the district court must expressly state those reasons on the record. Furthermore, we hold that absent an affirmative, compelling showing by the State that the alleged victim is in need of protection, the second Keeney factor favors examination of the alleged victim by a mental health expert employed by the defendant.
*451 Evidence of the crime has little or no corroboration beyond the testimony of the victim
Although the evidence against Griego is sufficient to sustain a conviction, it is hardly overwhelming. Although some corroboration other than the alleged victims’ testimony that a crime occurred exists, the quantity of such evidence is de minimis. Therefore, this factor too favors allowing Griego’s expert an opportunity to examine John and William M.
There is a reasonable basis for believing that the victim’s mental or emotional state may have affected his or her veracity
The veracity of John and William M. is questionable at best. First, at the preliminary hearing, Griego’s counsel asked John M.: “Can you remember which things people told you and what you remember all by yourself?” John M. replied: “No.” This reply unquestionably places John M.’s veracity in issue.1
Next, a multitude of defense witnesses who knew John and William M. testified that they habitually lied.
Finally, bad feelings between Griego and John and William M.’s father existed after Griego broke the father’s jaw and refused to assist in paying the medical bills. This animosity calls John and William M.’s veracity into question.
“Numerous jurisdictions, as well as general notions of fairness, dictate that failure to provide equal access to expert psychiatric witnesses prejudices the defense.” Lickey v. State, 108 Nev. 191, 194, 827 P.2d 824, 825 (1992). Having balanced the Keeney factors under the facts before us, we conclude that Griego is entitled to a new trial on the counts of conviction involving John and William M.
CONCLUSION
We affirm the conviction on count XXI,2 reverse the convictions on counts I, II, V, VI, and XI through XX,3 and remand for a new trial on the reversed counts only.4
Springer, J., concurs.We also question the reliability of John M.’s testimony insofar as the interview procedure used by the State may have been suggestive. See Felix, 109 Nev. at 174, 849 P.2d at 236. This concern would have been either confirmed or dispelled had the district court given Griego’s mental health expert an opportunity to examine John M.
Lewdness with a minor, Robert C.
Counts I, II, V and VI charged sexual assault with a minor under fourteen years of age (John M.). Counts XI through XVIII charged sexual assault with a minor under fourteen years of age (William M.). Counts IX and XX charged lewdness with a minor (William M.).
The Honorable Miriam Shearing, Justice, did not participate in the decision of this matter.