For our trip in Sept. I plan on doing the following as many have stated above -
Make sure to keep on the water, I often have to "make" my kids drink even if I ask them if they want some water and say no. I just tell them that its important if they want to keep playing to drink even if they dont feel thirsty.
I think for our kids def. we will have some nice "bucket" style hats / and something similar for us if we feel the need.
I might pick up some of the frog-togs / cooling bandana type of deal just to have depending on the heat.
Sunscreen - It has been very sunny and warm where we live and I am always putting sunscreen on my kiddos when we are outside - The spray kind is just easy and they dont mind it so i always have a can in my camera bag / stroller ect.....we will be getting some from costco before we leave
Like many have said also........take the middle of the day to get in some pool time / naps / smoothies and A/C ect......I remember being a kid and we always did the mid-day naps, sure its going to be hard to make my kids come back middle of the day but the pool will be a nice break. We dont want to feel rushed when we are there, my wife's parents would always just go-go-go all day then leave the park and not go back in the evening. We are not going with that approach.

This is one of the main reasons we chose the DLH for our trip also.