I would not recommend doing another park with two very young children. No matter how "small and cheap", they are still VERY pricey and you WILL have to pay for the youngest. Disney is a huge park and the Magic Kingdom will take you an entire day. Epcot will be too much for them and as others have said Animal Kingdom is very cool, but lots of walking.
Just a thought... Downtown Disney is a lot of fun and I don't think you have to pay to get into it. It's on the compound, but it's not a theme park. Tons of shops, restaurants and things to see and do. The Lego store is very cool!
Medieval Times is a cool restaurant/show. It's kind of cheesy, but if your son likes "Mike the Knight" or is into that kind of thing... it's fun. You eat in an arena and watch a jousting match complete with knights, princesses, crowns (you get to wear one), "medieval chicken dinner" and serving wenches. No idea what the cost is now, but you can easily Google it.