T.H.
Just wanted to say that I have a friend who has Celiac and she eats at Subway with no problem. You have to be slightly proactive and ask them to please change their gloves (she's never had anyone be rude about it) and then just have them put lunch meat/cheese, veggies, whatever, onto a wrap and give it to you that way. I've even asked for them to get out a new knife or whatever when I was picking up a salad for her. So, if your little one eats lunch meat and stuff, you could actually make it happen. Other than that, I have no adivce for you...I'm sorry! ;) I also agree to try and take as much stuff as possible. Since I eat out a lot with my friend I do know that a lot more restaurants are understanding and even catering to clients with gluten intolerance, so you may just start asking for gluten free menus and see what happens.
I do think thought that since your mom is sitting with them it will be a lot easier!! I would say bring a ball, maybe a balloon, some books, drawing stuff and things that make noise when she presses buttons. Since there will be an adult back there it will be easier to help her with some play time. Hang in there, I'm sure it won't be as bad as you think!! :)