HI Sue and welcome!!!
We'll be there in August as well, staying at the Polynesian.
The one thing that SAVED us last August (well, aside from trip insurance when Tropical Storm Faye arrived) was a bunch of Misty Mate Misters. They're the size of an electric toothbrush, and we were able to cool off whenever we needed to. I saw some the other day at BJs warehouse, for 3/$15-- an absolute bargain!!!
Also, have you checked out the DIS-abilities board here? There are probably some people there who can give you specific pointers on how to make your kids' trip even more special.