Kids-no Kids?
If this was a no kids trip I would say later in August or early September would be less crowds. By then, all schools should be back in session. All passes are unblocked, so the weekends and some evenings during the week may see some heavy traffic, but during the day you should see some pretty short wait times for rides. Weather can be pleasant to downright hot and sticky-but expect at least upper 80's to mid 90's most years.
That being said, early August is pretty good too. Our summer trips are usually early August. Crowds are really not that bad, just hit the park for rope drop and you can get a lot done before the crowds start picking up around 11. Go back to room and rest during hot part of the day, come back for parade or show, or more rides in the evening. I think end of August through September is still going to give you hot to very hot weather.
Either way, have a great trip!