I think a child is incapable of being able to discern somebody who has good intentions from somebody who has bad intentions. It´s very difficult as an adult to make that determination. You are unsure of what somebody wants when they start to talk to you. It´s not unreasonable that a child won´t. So the best thing to do is not to prepare them for trying to discern one person from another, but rather trying to be cautious with everybody they talk to.