Yes. Police may question a witness. Age is not relevant. However, there is no requirement that your child must assist law enforcement.
Circumstances may change if your child is subpoenaed to testify in court. If you are concerned that if your child if forced to testify at that trial or hearing, that he may incriminate himself, your child can avail himself of the 5th Amendment. You should seek the assistance of an attorney if that situation were to occur.