The problem the critics are having has nothing to do with having a 20 something year old playing a child, in fact that is barely mentioned. She is actually the one who the critics seemed to like in the show, so i guess that put's that to bed. If you feel or don't feel for her it's not because of her age it appears it's because of the script.
It was always going to be stripped down, putting some of the stuff on stage that they did in the film would just come across as campy, no matter who well it's done. The problem with this one appears to be the constant addressing the audience and the over analysing every moment by the characters.
Stripping it down and making it a story about good and evil and where evil lurks is a wonderful idea, but the story has to build organically rather than actors telling you what to feel.
Hopefully they can fix the problems and loosen the piece up a bit and just let it tell the story, otherwise i would suspect it will be put to rest out of town.
Namo i love u but we get it already....you don't like Madonna