The summer months in Central Florida can be extremely HOT! If you have small children, I would consider taking breaks throughout the day to cool off by getting wet! There are so many ways to "get wet" while at WDW.
2. Ride a "Water Ride." There are lots of rides that are in boats on water, but I'm referring to the rides that get you wet: Splash Mountain at the Magic Kingdom and Kali River Rapids at Animal Kingdom. (Or just stand on the bridge by either of these attractions and see if you can get splashed!)
3. Play in the "water fountain fun" in the parks! Downtown Disney Marketplace offers a great place to splash around in water fountains. At Epcot, you can splash in the fountains between the Fountain of Nations and the World Showcase. You can also play in the "jumping water" outside of Imagination. At Animal Kingdom you can play in fountains near Kali River Rapids and use the elephant squirters to get the riders wet on Kali River! I don't recall any water features for children at Hollywood Studios or Magic Kingdom. Just remember to pack a towel, a change of dry clothes and even a swim diaper and swim suit!
4. Return to your resort and take a dip in the pool! Or better yet, resort hop and try other pools!
5. Spend the day at Typhoon Lagoon or Blizzard Beach.
6. Last, for the older, daredevil types, you can rent motorboats, go parasailing, or waterski on Bay Lake!
What am I forgetting? What is your favorite way to cool off with water during the hot summer days at WDW?