We were there mid-late August and followed many of the tips already provided.
Here's what we did:
#1 rule - stay hydrated!!! Prior to the trip, I purchased camelbaks for all of us (kid-sized and adult sized). I had no interest in being the water carrier for everyone and the kids bags were light enough that they were only carrying a few lbs when filled We filled them up every morning and the kids were more than willing to drink the water because we added propel mix. We'd refill the pouches before we went out each evening. Excluding other beverages with meals, etc, we had an average of 2-3 liters/day.
#2 - Get the park at opening and leave everyday by 1-1:30. We spent 2.5 - 4 hrs each afternoon relaxing, laying low, and staying cool. As much as we had wanted to go mini-golfing one afternoon, we all realized that we needed a break from the sun and heat!
#3 - While in the park, we pretty much tried to get to something indoors (A/C) by every 3rd ride. Those waits and rides inside did a lot to keep us in good shape.
We also had the misters that you pump (got ours from
Walmart) and they helped keep things more comfortable.
The right clothes can help too. Finally, definitely no dark shirts! My kids have a few duo-dry/quick dry shirts (Champion brand from Target) which are lighter-weight and better ventilated than 100% cotton. They also had light-colored hats.
I thought it was going to be so much worse, but we were definitely ok. We'll take the hot/humid weather and lower crowds anyday over the Thanksgiving crowds (the only other time we've been able to go)!