I mentioned some in a post within the last week or so:
MK has Tom Sawyer Island, a playground under the railroad station by Splash Mountain and an indoor playground at Dumbo. Epcot has the most - play areas after Journey into Imagination and Mission SPACE (enter through the exit), the aquariums after the Seas with Nemo, the hedge maze in the UK pavilion, the actual cars after Test Track, etc. They also have Kidcot stations where he can color at all of the World Showcase pavilions and the miniature trains in Germany. AK has the Boneyard playground, which is awesome, and my son loves taking the train to Rafiki's Planet Watch, where you can brush the goats at the Affection Section. DHS is tough... there's characters and the Disney Jr. Live on Stage, but we can't spend an entire day in that park with him at this age. Next trip we are only going there for lunch at Sci-Fi (his request) and then hopping to another park."
With 5 year olds, your best bets would be:
MK - Tom Sawyer's Island (skip the little playground by Splash Mountain - they will be too big)
EP - ImageWorks (after Journey into Imagination) and the hedge maze tucked back in the UK (they can run around, you can see the tops of your heads, and if you are so inclined, enjoy a nice cold beer from the stand outside Rose & Crown while they get energy out). The kidcot stations are also a great little break while you are going around the World Showcase, as they can sit and color at each one.
AK- The Boneyard - there are slides, areas to climb, etc. Also, after you enter, make sure you cross over the bridge that goes over the park walkway. They have a "dig site" sand pit with shovels and buckets to use. It's not regular fine sand, so you can easily wipe it off, and it's shaded. There are also misting fans on when it's hot. A train ride to Rakiki's Planet Watch is also a nice break, and after a short walk they can go brush the goats at the Affection Section area (probably one of the cleanest petting zoos in the world).
DHS - nowhere really to play there