Hi Diane,
Really the only actual "splash play area" in any of the theme parks is
Casey Jr. Splash N Soak Station in Fantastyland at Magic Kingdom. My then-2 year old had an absolute ball this past June while his older siblings rode Goofy's Barnstormer just across the way. It's circus theming is absolutely adorable and a great place for the little ones to cool off and run around while you take a quick break. There are areas like the jumping fountains near Journey to Imagination where the kids might enjoy a little splash, but it isn't an actual splash play area.
There are other play areas in the different theme parks such as the
Honey, I Shrunk the Kids Movie Set Adventure at Disney's Hollywood Studios or the
Boneyard in Animal Kingdom. These, too, can provide a welcome break from standing in lines while the kids run off some energy and have a blast. There are multiple shaded areas around these for parents to sit down.
Another suggestion that my kids love, but parents might use their own personal discretion, is purchasing the souvenir squirt bottle fans that they sell throughout the Walt Disney World theme parks. These provide a refreshing spray of water to keep cool during those hot days. Again, this is simply a recommendation.
Keep cool and have fun!
~Allison L