Hi, Katie! One of the best purchases I ever made were the clip on fans I bought for my girls when they were still in strollers. The fans attach with spring loaded clips to the stroller, and are kid safe. They didn't have them very many places when my girls were in strollers, but you can now find them at Walt Disney World and at places like
www.rightstart.com .
Keep the girls well hydrated - yourselves as well. Water works best, but whatever you can get them to drink is going to help. Use sunscreen very liberally - they are really going to need to be well protected from the Florida sunshine! If you can get them to wear little hats and sunglasses, that can keep their noses sunburn free!
I also make it a point to get my girls inside if we've been outside too long. If we have had to wait outside in a line for a while to ride something, I usually ride the ride, then head to a gift shop to soak up some air conditioning. We also might go from one outdoor thing, like Dumbo the Flying Elephant, to an indoor thing, like Mickey's PhilHarMagic.
If you can get to the parks when they open (even taking advantage of extra magic hours in the morning if you are a Walt Disney World resort guest), you can get at least a few things done before the hottest part of the day. That's a good time to do some of the outdoor attractions. Use the Baby Care Centers, located at each of the theme parks, if you need a break during the day and don't wish to return to your resort.
Have a wonderful vacation!