When kids surprise us, it’s not just because they’re doing something in a mature way. It’s because they’re doing something that most adults can’t do. This was true of the actors in Mill Mountain Theatre’s “The Wizard of Oz: Young Performers’ Edition,” which played on the Trinkle Main Stage at Center in the Square Saturday afternoon. The production featured students from the Mill Mountain Theatre Conservatory, an educational program that teaches students from kindergarten to 1