Take advantage of Early Entry mornings so you can leave the parks at the hottest part of the day. Either have lunch before you leave the park or in your very cool, inviting (make sure you leave your air conditioner on in your room before you leave!) room. Take a swim, watch TV to relax in your a/c room or head for a water park. Just stay out of the heat in the middle of the day.
And take a water bottle for every member of your family and drink, drink, drink. You could pack some Kool Aide in a mini M&M container in your belly belt or back pack. Refill often at any drinking fountain. That way the kids won't bug you too much for a soda!
Oh yeah - an ice cream break (indoors so it doesn't melt so fast) is a great way to beat the heat!
Have a great trip!