I've been a vegetarian since I was 12. I'm 36 now, so that's almost a quarter century of vegetarianism.
If you google "vegetarian kids" you'll find lots of great sites. Several of the best have already been mentioned, but I would add http://www.vrg.org/family/kidsindex.htm
If you're looking for recipe books, I would suggest "The Vegetarian Lunchbasket" , "Kids Can Cook: Vegetarian Recipes" , "Cooking for Your Vegetarian Kids" and "The Teen's Vegetarian Cookbook".
I'm not sure what to do about Grandma. If her concerns are nutritional the resources suggested in this thread should ease her mind. If she has some irrational prejudice against vegetarianism, then I guess you just have to be firm with her. Your daughter's body belongs to her, and no one has the right to do anything to her against her will, including force-feeding her. If Grandma refuses to respect your daughter's right to control her own body then I would think that your daughter has the right not to be alone with her grandmother. But hopefully the situation isn't as serious as that. Several of my aunts were worried about my decision to go vegetarian, and would try to sneak meat into my food, but my parents stood up for me and everything was fine. After a few years it became obvious that a) I wasn't going to change my mind, and b) I wasn't going to die of malnutrition, so they got used to it. I usually bring a vegetarian dish to family events so the host doesn't have to worry about catering -- at first some relatives thought that was a bit weird, but now they ask me to make sure to bring enough for everyone :-)
Best of luck to your daughter!