In all fairness to both the directors and the poor kid stuck with playing Peter Pan, the boy could sing. Decent voice, hit most of the right notes, he just couldn't sing in public. Poor guy had stage fright like you wouldn't believe. While my sisters think the primary deciding factor is whether the kid can fit into the costume, I do think the directors try to assign the roles based mostly on talent. On the other hand, they do have a rather limited pool to pick from.
P.S. Sorry for the delay! Again! I've been having a really hard time finding enough energy to do anything outside of work. Even while at work it's a struggle to keep going. If I could, I'd take the next week off and just sleep.