Crowds is always a relative thing. They do get smaller after the holiday itself, but if you want to be sure it will be lower, I'd wait til the following week. Some folks will keep their kids out for a couple of days of school, but by the following week, the number of school aged kids should be much lower.
Disney has recently been offering free dining during this time, and that does increase crowds to some extent. Everyone likes "free"!
And you didn't ask, but please be aware that even though it's September, it's VERY hot still. You'll still see temps in the mid 90s with high humidity. Great pool weather!