Hi, Kim! There is no minimum purchase per guest at the table service restaurants (other than the buffets/family style restaurants where it is a set cost per person.) So, you may opt to go for just drinks and appetizers, or whatever you are in the mood for!
You may want to check out the menus for each of the restaurants at
www.allears.net. You will get a better idea of costs of meals, locations that look good to you, and menu items that your boys will enjoy.
Don't overlook some of the great quick service options that are reasonably priced: the always popular smoked turkey legs, Epcot's Yakitori House in Japan and Cantina de San Angel in Mexico, Pizza Planet in Hollywood Studios, Cosmic Rays in Magic Kingdom, and Flame Tree BBQ in Animal Kingdom. These are all counter service locations that you might find to be fun and affordable!
Have fun planning your Disney vacation!