Seems evey time we go it's some kids birthday and that's who they pick. (they usually have one of the free birthday pins on that you can get from Town hall) My son would love to do this too someday.
Basically, it's about a 15 minute show where Merlin comes out to the Sword in the Stone and has to find someone to be King/Queen. Takes place in front of the Carossuel <sp>. It's a cute show. If your kids saw the movie and liked it they would probably get a kick out of it.