Hot hot and hot. But - doable

I agree with the advice about getting out early (so many plan on doing it but don't by the time they eat breakfast and get everyone moving, so it is also great for riding rides before the real crowds get there), getting lots to drink and taking sunscreen and heat breaks.
July and August are even worse for sure. We went to the Star Wars weekend in May last year and it was close to 100 every day and I think you multiply that times 2 or so because of the Disney blacktop (atleast at MGM) and you get the picture

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There are places in each park where you can cool off (shows or restaurants, movies or just shady nooks). Take advantage of them and don't try to push yourself too hard and you will love it.
If you are staying on property then try out the pools or other entertainment at Downtown Disney (now ask me how many times we have actually gone to see a movie, etc ....

I am a park commando, but trying to change my stripes as we just joined DVC and we go down when it is not so hot and the park hours are shorter, so "all day" is not like all day in the summer). My children still nap in the stroller, so sometimes (if we are not going back to the hotel) hubby and I just find a cool place to sit and enjoy an icecream or some cold water while they nap. We all feel refreshed and ready to go again once they wake up.
One last thing....those fans with the water bottle attached to them...our sons both have one and they LOVE them and they do work. People around us are always asking for a squirt! It is a quick refreshing blast of mist just when you need it most.