Hi Maryann,
Thanks for your kind words about the Disney Parks Moms Panel. We love helping guests to plan magical vacations! I am so glad that you are finding the information here helpful.
While it is always warm in Florida, the coolest months are December, January, and February. January is typically the coolest, with high temperatures around 70 degrees Fahrenheit on average and lows around 50 degrees Fahrenheit on average. January also has the benefit of being a month that typically sees less crowds since it is right after the winter holidays. Walt Disney World marathon weekend is often in January, so you might want to avoid that weekend for heavy crowds.
I would also like to refer you to the Walt Disney World page for
Services for Guests with Disabilities. This page lists many of the ways that Walt Disney World would be able to support the young adult with whom you are traveling.
Have a great time planning your magical vacation, Maryann, and please do not hesitate to ask if you have any further questions.