I totally agree with westjones. That site gives a lot of great information!
The thing that always killed us when planning was the ice cream...Those mickey bars are all over the parks and are 2.50 a shot.
Even though you think you may not buy them, there's something amazingly tempting about them...and unfortunately they're pretty pricey for a family.
We found that at Epcot, we could get similar decent food at the stands that were adjacent to the restaurants. Instead of paying the sit down prices, we would often buy the food from the vendor and find a nice table outside and usually there was music to listen to or something going on. This saved us a bunch at lunchtime.