There is a play fountain in Epcot, in the main path between Future World and the World Showcase (the big path that leads through the middle to the lagoon). It's on the right-hand side as you face the lagoon.
The closest thing I can think of in MGM would be the "Singing in the Rain" umbrella somewhere on New York Street. It may or may not be working, however.
There are some misters in Tomorrow Land in the MK and there used to be one in, I think Africa in the AK but these aren't really fountains - they're more intended to cool off people too hot from walking on massive amounts of concrete. Off hand, I can't think of any "play" fountains in either park.
Mur!