Hi, Melissa! September is a good time to visit as you will see some of the smallest crowds during this time of the year. Kids are back at school, and many families do not want to pull their kids out this early in the school year.
Remember, though, Disney is currently running an offer for
free dining during September, so this will increase the amount of guests booking a WDW vacation. Keep with a plan of getting an early start and having Advance Dining Reservations. Do not be scared off by this, the crowds will be manageable. I just don't want you to have ideas that you will be alone on Main Street and not need to plan ahead!
September is still very hot in Florida. Plan on mid/upper 80's. Bring sunscreen and drink lots of water to stay hydrated.
Good luck planning your September vacation, and I hope you and your family have a great time!