It's hot. but managable. I've gone the first wek of Sept. the past 2 years and am going that time again this year.
FL is hot and humid in the summer..but I didn't find it umbearable. First of all the parks weren't that crowded so you didn't have to wait long for rides and there weren't mobs of people which help with dealing with the heat.
Disney has lots of air conditioning which also helps. I find great attractions to rest and cool off on are Hall of Presidents, It's a Small World, Universe of Energy, the great movie ride - the rides are indoors & air conditioned - usually not long lines and they last long so you can just relax an cool off.
We stay in a villa and order cases of water from garden grocer. We freeze a bunch of bottles the night before and then carry them in the parks. They help keep us cool and as they melt we have cold water to drink.
Afternoon pool breaks also help. You can get those spray fans if you want. One year I found these items on
amazon - they look like bandanas - you soak them in ice cold water the night before - there are these little beads in them that absorb and hold the cold moisure and then you tie them around your neck and they help keep you cool.
Unless you have a REAL aversion to heat or anyone in your family cannot tolerate it, you will be fine. You just go at a slower pace if need be - take a few more breaks to cool off and enjoy yourself. If you have a small child, the child care centers are GREAT places to cool off and rest. We used the one in MK a few times. In addition to child care facilities they have a room with a TV and chairs and you can sit in there with your child and cool off and give yourself and your child a break from the heat.
I go when I go because it's less expensive and less crowded and my son is only in Pre-K so we can go anytime of year. When he goes to school I figure we'll be taking August Disney trips...and you know what, I don't mind at all because I think the hardest part about the heat is if you push yourself to do and see everything. If you take your time and pace yourself, you'll have a great time and be fine.
Have fun.