We've gone a couple of times in September (we'll be there again this coming Sept!). Definitely get up & going early before it gets too hot. You can get so much done before the parks fill up. Then you can either duck inside a show, restaurant, or head back to the hotel to swim & rest during the hottest part of the day. That's what we alwas do & it gives my little ones nap time while the older ones chill in the pool. We also found these little clip on battery operated fans that we hook to the stroller. They're bendable and have foam blades so the don't hurt, but they keep the kids cool. The older ones usually have the spray misting fans.
Just go in knowing you can't do it all, even in a week, so plan ahead what you definitely don't want to miss and leave yourself a bit more leisurely pace.
You'll love it (and might be hooked!). Have fun