Are you disturbed by a behavior you see either in your own child or another child? Do you wonder if it constitutes a symptom of problems that require professional help?
Even in such a situation, you are best to respond as you would to normal childhood sexual play. You will have time after the children are busy with something else to think about what you saw, to speak with someone you trust or to consult with a professional.
If the sexual behavior is truly pathological, then the child should get professional help. However, if the troubled child is not your own, you cannot force the parent to pay attention. In this case, you may be well advised to report the situation to a child protection social worker who, hopefully, will know how to handle the situation diplomatically and sensibly.
If the troubled child is a friend of your son or daughter, then make sure they are together in supervised play only and do not let your child go to his or her house alone.