In each of the parks, there is a playground. 100 Acre Forest - across from the Winnie the Poo Ride in Magic Kingdom. Hollywood studios has one also and Animal Kingdom too. THat would be perfect for your kids. They can run and play. Also in Magic Kingdom, the Tom Sawyer Island is the best kept secret! You take a raft ride over to the island and than explore on foot. It is a nice safe envirnoment. They have a fort, secret tunnels, fun bridges to cross. At Hollywood Studios, they have a show that you sit on the floor and watch winnie the poo, Little Einsteins and Handy Manny. Your kids will love it. But before you go you should have them do "homework" and make sure they are familiar with the Cars movie, Monsters Inc. Hanny Manny, Mickey Mouse Clubhouse, Peter Pan etc.. It is more exciting for them when they see something they know. In Hollywood Studios, they also have Luigi's garage with McQueen and Mater there. You get your picture taken with them. Very cute. I probably would not go to Epcot. There is not much to do for young kids.
In response to other parents, yes they won't remember the trip, but you are building a foundation for them of good feelings.