We went with our 3 kids (6, 4, & 2 at the time) in Sept. 2007. We left the Sunday before Labor Day and stayed through that Saturday. Crowd-wise it was great. We hardly ever waited to get on a ride, even the most popular ones. My 4-year-old son went on Soaring 4 times without waiting on line! As far as the weather, it was sweltering hot. We drank more water than you would believe. On the plus side, we saw almost no rain except one brief shower. We survived the heat mostly by getting to the parks first thing in the morning, staying til only about 2, then heading back to our hotel for some down time of napping or swimming. Then we'd change clothes and go out to dinner and maybe a few more rides or fireworks. Worked for us. At 8 & 4, your kids can probably handle it. Remember too, some of the attractions are air-conditioned and/or shady. All in all, I would go that time of year again.
M.