Pinocchio's Village Haus in MK. Mac and Cheese is very good, as are the pizzas and fries. For your eldest, I highly recommend the Figaro Fries.
Also, I don't know where you're staying but we save a good bit by eating cereal in the room for breakfast, and the occasional sandwich for lunch or dinner depending on if we go back to the room to swim in the afternoon. At the values you'd need a fridge for this - $10/day, way less than you would spend on a meal.
I also make my own trail mix to take to the parks. I mix vanilla creme mini wheats, m&m's, pretzels, cheezits and peanuts together in quart size freezer bags, and put them in a backpack with 2 water bottles per person (1 per person frozen). DH carries the backpack and appreciates the "cold pack", LOL.
If you are flying here, and don't have transportation, I'd maybe take a cab to Publix and stock up. Their weekly sales begin on Thursday and end on Wednesday, FWIW.
Oh, and a quick afternoon snack at Epcot is Norway's lefse - very economical. I also like the Boulangerie Patisserie in France.
Hope this helps! Let me know if you need anything more specific.