I think the subject matter is a little heavy for a 12 year old but the show does an excellent job of presenting it in a way that moves you but doesn't allow it to bog the entire experience down. The subject matter involves domestic abuse, adultery, some salty language and a few sexually suggestive moments. I have never seen the movie and someone commented to me that the movie was awful and not worth seeing. Now that I have seen Waitress twice, I don't want anything to spoil the experience I've had. According to this person, the movie is nothing like the Broadway production, so I'm not sure that watching the movie will answer your question about whether this show is age appropriate.
Bottom line is that you know your children better than we do, so if you feel they are mature enough to understand and not be upset by some of it, then its a wonderfully entertaining production.
The only review of a show that matters is your own.