Wouldn't the best way to approach the boy who wants to be a girl be to ask the boy why he wants to be a girl?  Perhaps he just likes dolls or wants to be a dancer.  In that case, gender theory would suggest that it is okay for him to play with dolls and be a boy.  That is the idea behind gender theory.  Of course, if the child has gender dysmorphia, then that is a more serious conversation.