Regarding children, that question would probably be pointless... Can you even imagine them saying 'no'? A parent is much more likely to ask an adult "Were they good?"

It can mean many different things depending on the context and circumstance. Example 1.A boy goes to play at a friend's house.Mother picks him up and says' Were you good?" it means "Did you behave well?" 2.A person plays piano at a concert. His friend did not attend and asks him after the concert "Were you good?" it means did you play well at the concert?" There are lots of other possibilities so without knowing the context I cannot answer your question.