I would recommend November thru January, avoiding the holidays and weekends surrounding the holidays. This would mean avoiding Thanksgiving week, the two weeks surrounding Christmas through New Years, and MLK weekend. If you visit in early December, you can also see the Christmas decorations in the parks and resorts. All the other times in these months should have very low crowds. Another benefit to this time is that the weather is cool and makes touring the parks a lot more tolerable.
September also has very low crowds, but the downsides to Sept are hurricanes and it is still very hot.
Good luck in your planning.