I have been there in every temperature range from the 30’s to the upper 80’s. While you can almost count on cool weather in January and February, December is more of a wild card. A lot depends on what is happening up north and how strong the early winter cold fronts are.
Definitely bring layers. I live here and when staying at Disney in the winter I take a medium weight hoodie and a fleece jacket that I can layer (fleece over hoodie) or wear separately. I bring gloves and a scarf. I bring half long pants and half shorts or capris. I bring one long sleeve shirt, but mostly short sleeve as they will either be worn alone because it’s warm or be the innermost layer. Bring a swimsuit if you like the pools (they are heated). Bring a rain poncho to use not only if it rains, but as a wind blocker on a cold windy day (it will be another layer on top of your jacket).
Our weather tends to cycle in the winter. You get the first blast of freezing weather, then it warms up a little day by day and just when it feels great, the next cold front will come through. Definitely check the weather a few days before you leave to get a good idea of temperatures and rainfall. Unlike our fast moving summer storms, it is not unusual for it to rain for a full day or even a few days in a row if a front is slow moving.