Our family went to Disney for a week each of the last two years in August. Hot hot hot!
Here's what worked for us:
1) we bought cases of water from Garden Grocer and had them delivered to our resort. Much cheaper than buying it in the parks.
2) Dh and I each carried a backpack and made sure to bring plenty of water each day.
3) PLENTY of sunscreen
4) loose cotton clothing
5) misty spray fan thingy
6) imperative to get to the parks early. you get to comfortably see sooooo much more in the first two hours than if you get there at 10 or 11. I've done it both ways. It's true
7) my children just couldn't handle it past noon. We'd get there at 9 and do so much and be OUT of there by noon when the mobs came in and the heat was dreadful.
8) back to the resort and have the time of our life in the pool. This was my children's favorite part of the day
9) Then we'd go to our room for a rest after pool time and ice cream
10) freshen up and get ready for dinner.
11) after dinner it's starting to get cooler and we might stay at a park we're at, or do downtown disney. Usually we're done with the parks for the day though.
This is just our way of dealing with it and it has worked well for us.
If you have a little girl that wants to dress like a princess in those blazing hot outfits, put her in it and bring a change of clothese to the parks so after your cinderella breakfast or meeting the toontown princesses or ariel, she can change.
Also, last summer, thanks to DisBoutiquers, I learned to make adorable calico like princess sundresses that she could wear and be dressed like a princess but not sweltering. There are also stores on etsy where you can buy them.
Don't have much advice on the boys since my boy is so easy to dress. 7 shirts and 7 shorts from target. bam.
Hope this helps
We went the last week of August both years and crowds were quite manageable!