We went last August during the same time period. We are going again next week. Yes, it is hot and yes, it is humid. But, that did not deter our experience one bit, chiefly because that is the time frame we were able to go, we had an amazing vacation package, and were prepared for the heat. My concern for you is more of whether you will get FPs that you want, as I think that helps us to have a better time when we get to the parks relatively early (not necessarily rope drop), and then take a break, then hop to another park where we use FPs. Also, as an aside, I did not find the crowds to be unbearable last August. I have had a tougher time at Christmas time.
I guess what I am trying to say is this: if your family has decided that that is the timeframe you are able to go to Disney, see what packages you can find. If it continues to make sense, go ahead and book the package...then scour the boards for hints on how to deal with the heat. I do not necessarily think that it is unbearable or a deal breaker. Managing expectations is the key to any nice vacation. If, going in, you know it will be Hades hot, like walking on the sun hot, then you can go from there. Drinking lots of water, taking breaks, enjoying the pool, taking in a show, eating a nice cold treat, slowing it down to ride the train around the park, perusing shops or countries in the World Showcase. Your Disney trip is what you make of it, regardless of the weather...seriously.
I hope you get the chance to go and have a great time.