Oh my goodness, Carolan, considering your vacation will be two weeks
long, you have plenty of time to visit all four of the theme parks, plus
both water parks if those hold any interest for your family. I can
tell you that all of the theme parks offer something unique for
children. Yes, even Epcot. In fact,
Innoventions is a favorite for my
four kids (who range in age from five to seventeen). I can't convey enough how strongly I feel that you would truly be missing out if you chose only to visit Magic Kingdom during your two week stay. Each park offers something different to explore and I do believe that your children will enjoy each of them. Because the weather will still be quite warm in September, you might also really enjoy both
Blizzard Beach (we LOVE
Teamboat Springs) and
Typhoon Lagoon as well.
DisneyQuest, on the other hand, is not a must do, in my opinion.
As far as crowds in September, I think you're in for a pleasant surprise. Historically speaking, crowd levels at that time of year have been very tolerable. It's going to be such a great trip!