Hi there, Lacy!
September - particularly after Labor Day weekend - is a great time to visit
Walt Disney World. With so many students back in school for the fall, crowd levels, while not necessarily "low," are certainly diminished in comparison to the high travel season in the summer.
A number of non-affiliated, third-party Web sites use a combination of statistical analysis and historical data to predict crowd levels for the future. The one I use that demonstrates a great deal of accuracy is TouringPlans.com -- give that one a look and maybe their information will help you determine exactly the dates that work best for you all.
Two days per Park sounds perfect. You might find, too, that you dedicate only one day to any given Park;
Disney's Hollywood Studios, for example, is certainly doable in a single day. That will buy you extra time for shopping,
DisneyQuest, a day of lounging around the pool ... whatever works for your travel party.
Have a wonderful time!
See you later, celebrator,
-Deborah B.