OP you know your DDs best but if you're just going for lunch/tea party and not planning on buying anything then I would really caution against going. This store is like going into the World of
Disney store and telling the kids they can't buy anything.

It's like a little girls dream. There will be displays of dolls, doll clothes, matching girl clothes, stuffed animals, books, hair stuff, etc. And if I remember the Chicago store correctly, the restaurant isn't on the ground floor. You have to walk through all this stuff to get there. The restaurant is a nice addition to the store but the main purpose is for you to buy stuff!! And they will try to sell you stuff from the minute you walk through the door.
I believe that even the most well behaved little girl will want to walk out with a nice little shopping bag of something. Might I recommend one of the AG pets such as Coconut. They even have play sets for the puppies.
Good luck and enjoy your lunch.