We were there in May one year and it was around 100 Fahrenheit, which we were told is unseasonably hot. I found that it feels way hotter in the parks than it does outside the parks. If we were ever going to return to DLR during the summer months, we have our expectations set based on that experience. It is doable - you take pool time, you enjoy the inside rides, you drink LOTS of water and I also found wearing a good hat makes the world of difference when in the blazing sun is beating on the concrete ground!
I do agree with the lack of humidity too - there is very little there compared to the east coast - so it doesn't drain your energy nearly as much.