Sure they can call the police. What the police will do is anybody's guess. In many jurisdictions, trespass to land is just a civil tort, not a criminal wrong. I expect even if it IS a crime in the jurisdiction, the police would laugh at them and tell them to call the parents, or simply walk the child back home, and stop wasting their time. But you might want to pose that question in the Criminal Law forums just to be sure, or simply phone the police station and inquire what they would do in the situation.