It sounds like Lea Michele is making a "special appearance" and not actually playing a part in the show. They should have her play her original part, but I guess the other children would then have to be played by adults as well, which they didn't want to do. Or maybe she wanted to do something smaller than her original role.
I'm not sure why they have a standby for the "Little Boy" and not apparently for any other role in the show. The Little Boy's part is more prominent than the other children, but it seems weird to have a standby for one part only.