You didn't say where you will be: I am going to go with Magic Kingdom:
The family meal deal at Cosmic Rays---I've heard is a great deal.
1/2 Chicken and BBQ Rib Combination - served with Mashed Potatoes and a seasonal vegetable $13.99
most people say it feeds several people. You can also just order extra sides like fries for the kids
Or--My family usually orders kids meals for everyone. I find it fills me up.
Or take the monorail to the Grand Floridian to Gasparilla Grill & Games--pizza 16" Cheese Pizza $13.99
If you are taking in lunch and snacks--pack heavy on the snacks. We find we get so excited at Disney that we have to 'remember" to actually stop to eat--snacking all day works for us.
And no matter where you eat there is the extra bun tip I've read about:
Here's a clever trick light eaters can use at counter-service restaurants at Walt Disney World: order one double cheeseburger combo meal and ask for an extra bun. (The cashiers actually have an "extra bun" button on the register, so this is not an unusual request.) Then take one of the hamburger patties off the double burger, and put it on the extra bun. Most of the counter-service restaurants have a condiment bar where you can add any toppings you like to the two burgers. Split the fries and drink, or order an extra drink. This tip works especially well when you have two small kids who don't eat a lot. Thanks to rams408 for the idea.
Mousesavers.com will have more ideas for you:
http://www.mousesavers.com/meals.html#tips
HTH