Our first trip to WDW was in July. It was HOT!! But, if you are expecting that--you can bear it. I would suggest: mister fans, cool cotton clothes, Cool Max socks(these are great); washclothes for face; lots and lots of water bottles or small thermos of ice to melt throughout the day and lots of sun screen(sport/waterproof).
We always went to the park early, as early as they opened and by afternoon went back to the resort for a break--sometimes a swim, sometimes a snack and rest on the bed, usually a shower for everyone or sometimes a meal in a/c. Then, we were ready to head back and do it again in the evening! You absolutely cannot go without taking a break. You will tire out too quickly and things will get ugly--especially if some members of the family don't do well with heat. Plan to do shows or a snack break in the a/c in the middle of the day--just sitting through a short show in the a/c gets everyone rested up and ready again.
The temp. does go down with the sun--not a lot, but enough to notice the difference. We packed jeans and sweatshirts, but didn't wear them once.
ENJOY!!!! Even in the heat of July, our first trip to WDW made us lifelong lovers of Disney!