As a local, I agree with what one pp said -drink often, and drink before you feel thirsty. If you are already thirsty and sweaty, it's too late, you're dehydrated. Now, any fluids will quench thirst, but in the long run, things like water, gatorade, powerade, etc. are all better because soda will eventually dehydrate you by causing you to go to the bathroom more.
Aside from drinking a lot, take advantage of going into shows during the afternoon hours and just relaxing out of the sun in the air conditioning. You can also walk into the stores (most are connected) which are air conditioned and give a nice break. You can also wet your face and neck with wet paper towels and that will help cool you down.
With little ones, it may be easier to go back to your resort for a rest/pool fun during the early hot afternoon hours, then return in the cooler evening hours if your kids can stay awake, when it won't be as hot and the parks are more bearable. Obviously, do try to get as much done in the early mornings and evenings when it is cool!
Summer months like July and August are hot and humid!
