Both times my parents took us as kids to WDW it was middle of August. It was hot, yes. It was humid, yes. But we enjoyed every moment of both trips! Even when we got caught in a HUGE thunderstorm on the second trip...we still laugh about that till this day!
DH and I prefer to vacation in July. We are teachers and do not like looking forward to the end of the summer, but rather go in the middle of July. Give us a couple of weeks to get ready for our trip, buy any last minute odds and ends, ease into the summer, and still have plenty of time off after the vacation before school starts again. Just the way we prefer it, whether the trip is to Disney or somewhere else.
I HIGHLY recommend getting to the parks well before they open. It is more crowded on an EMH morning, but we took advantage of 2 of those mornings back in July and we still did not wait on any long lines. If you do it right...and by that I mean get there early and wait in line for them to let you in rather than waiting in lines at rides later...then it will be very pleasant and you will see so much before lunch!
We got to the parks and waited in line for 30-45 minutes before they let us in before park opening...on about 75 degrees, better than waiting 45 minutes per ride in 90 degrees. We did a lot before lunch, and did not leave any park later than 2PM. The hottest point of the day is 2-3PM, so we made it our goal to be out of there by then. Most times it was by 1PM. Lunch at 11:30, then maybe an attraction or 2, then leave. And we saw and did everything on our list and then some! So it's not like we missed out. We took advantage of the fastpass, mainly for DH who loves thrill rides and those seem to be popular and there are few of them to begin with at WDW. But he got to go on all of the ones he wanted to and even some he did not think he would get to!
Sometimes we returned to a park for dinner, sometimes not. We did not do any nighttime entertainment because we felt it was more important to keep the kids close to their regular bedtime since we were getting to the parks so early in the morning. They were 3 and 1 when we went. And they were also very pleasant throughout the whole trip.
So I highly recommend getting there early...sticking to a somewhat similar schedule to what you have at home, especially if you have small children with you...and take a break in the afternoon! We just went back to the room in the afternoon, relaxed, napped, watched tv, whatever...in the air conditioning and then felt great!