I understand - we have small kids too - and quite frankly, I can't stand spending the money on the sit down restaurants. In Magic Kingdom, I'm a big fan of Casey's Corner. It's literally just hot dogs, but they are really good. There's basically two kinds - small and large. We usually get 3 larges, and the kids (ages 2 and 6) will split one. We also like Pinocchio's (forget the whole name) - it's right by It's a Small World. Hamburgers are good, and if you order the double cheeseburger, you can get an extra bun for less than $1, and you and hubby can split it. Pecos Bill (Frontierland) is basically the same food - we go to either place depending on where we are at lunch time. There is a restaurant (I believe in Tomorrowland) that has roasted chicken that is supposed to be pretty good. Not sure which one it is though.
Can't give you much help with Epcot, as we don't usually go past Future World. We have ate at the Cantina in Mexico, and it wasn't bad, but not really anything kids are going to eat.
MGM, we do usually do the sit down restaurant here (Prime Time Cafe), but they've got Pizza Planet with the Toy Story theming. Not the greatest pizza, but it's not bad.
Animal Kingdom has Restarauntasaurus - basically hamburgers and hot dogs again.
If you use this link, you can see the menus for all the restaurants. It will notate which ones are counter service.
http://allearsnet.com/menu/menus.htm#ak