The playground the poster above describes is called the Boneyard, which is modeled on a paleontologist's dig site. This one has a second part, connected to the main playground by a bridge over the walkway. There is a pit filled with pea gravel here in which the kiddos can dig up mastodon bones with the provided pails and shovels. Our loves it and would probably spend all day there. I love the soundtrack that they play over the speakers here, campy old pop songs.
The Honey I Shrunk the Kids playground in Hollywood Studios is also very cool. Everything, from blades of grass to ants to ant tunnels to a spider web, is gigantic, as if you have been shrunk to about six inches tall. It is easy to lose track of an adventurous kid here because the tree trunk has multiple entrances and exits and three levels. It's naturally dark inside and only about three feet tall so adults have to scramble if you want to keep up.
I don't know of a playground at Epcot. Maybe someone else does?
There is (or was before Dumbo's relocation?) a small playground in Toon Town in the Magic Kingdom. It's near the train station. There is no playground per se on Tom Sawyer's Island, but it tends to be far less crowded than the rest of the Magic Kingdom and can be a good place for little ones to stretch their legs without getting trampled. I believe there is also a small playground at Splash Mountain.
Most of the resorts have a playground so you might search them out if you're there for dinner.
Have a great trip!