Hi,
We went mid-August 2005 when daughter was 7, and again in mid to end August this year, and had no problems with her in the heat/humidity at all - indeed, this year we generally walked around 7-8 miles each visit at each Disney Park.
I guess the key is in the planning. We had a couple of days at Seaworld first of all, and spent quite a lot of time sitting at the shows out of the sun, before visiting Discovery Cove.
We then had five nights at CBR. When in the parks, we didn't spend the whole day there (except for Animal Kingdom) but tended to go around lunchtime, which still gave us around 9-10 hours, rather than being there at first light for EMH and/or evening EMH (which we avoided at all costs!!)
Crowd wise, it wasn't too bad, even with the free
DDP - the only recommendation would be to make ADRs at the 180 day mark. We planned our days at each park using the various crowd calendars available, and then booked our Table Service meals around that.
Only unforseen problem was that they announced the Pirates and Princess Party nights after we had done our planning, and had to make some "tweaks" to our plans.
We also made sure that around every third day, we spent it a one of the water parks, and we took a long weekend in Clearwater after our first 10 days to visit Busch Gardens and also to chill out.
We then had a final 5 days back in Orlando, and again spent 2 days at theme parks, one day at water park, and final 2 days in theme parks.
I would also mention that many Education Authorities will be taking a tougher stand on holidays in term time from next year, and unless there is a valid reason for doing so (and cheaper flights/holidays will not be valid reason), then schools are being encouraged not to authorise holidays during term time.
Would also suggest that you do take a buggy for your youngest - then they can have a rest while you are still walking around the parks or if you are waiting for the evening parades/fireworks.
Hope this is of use.