I am from NY, so the August heat in Disney is REALLY hot for me. The first time we went, I thought before hand that I would be miserable the entire trip. Honest truth, it wasn't as bad as I had been expecting.
It certainly is hot, but if you are ready for it, you can manage. We always get to the park for Rope Drop. First thing in the morning, it isn't as hot, you can move a bit faster, and there are lower crowds, so you get a lot done. Towards mid day, we take a long, leisurely lunch, giving us about an hours break from the heat. Then, in the afternoon, we do a lot of the longer rides, shows, shopping, things that keep us in the air conditioning as much as possible.
Supper is usually our table service meal, and we expect to burn about an hour and a half on it, again a nice long break from the heat. After supper, it is usually starting to cool off a bit, and we will usually do a couple more rides, and partake of the evening entertainment.
Remember sun screen, you will burn, and that will make you miserable. Remember quick service restaurants will give you water for free. Drink often. Bring along flavoring packets if you aren't used to Florida water, the flavor can be unsettling. Pay attention to your body, and to your kids. If they are getting grumbly, something is bothering them. Take a minute, and find out what it is. Give them a break, a drink, a snack, what ever it is they need, and do the same for yourself. Finding a shady spot to sit down, have a drink, and just people watch can be just what your body needs. In Epcot, I particularly like the outdoor seating near the quick service in japan, up near the top of the water falls. It is a wonderful little area.