I go every year in December and the weather varies widely. Bring a variety of clothes, and be prepared to layer, especially early mornings and late evenings.
Last year, the week I was there (always the week before Christmas) the days were in the low 80's the nights in the high 50's. SHorts, flip flops during the day -- but fleece jackets and jeans at night.
The year before, it never got warmer than mid-60's during the day and high 40's (!) at night. That was a sweatshirt and jeans week the entire week.
Just watch the weather the week before your trip, and plan accordingly.
(Incidentally, I LOVE WDW when it is in the mid-60's rather than hot and sunny. It's the most comfortable weather. Put on a light sweatshirt and go. No sun block, no sweating, no worries).