Hi Christine,
You are visiting Walt Disney World at a positively beautiful time of year. My family has also visited Walt Disney World during the week between Christmas and New Year's Day, and I find the whole resort to be absolutely stunning when decked out for the
winter holidays.
As for packing for this time of year, I find layers to be a good idea. I prefer long pants along with short or long-sleeve shirt, as well as a sweatshirt or light jacket. My husband will wear shorts any time of year, so he brings those along anytime! I find that in the morning I need a sweatshirt or jacket, but can often put it in my backpack by mid-day, especially if the sun is out.
In December, the average high at Walt Disney World is about 73 degrees Fahrenheit and the average low is about 50 degrees Fahrenheit. I do not believe that it is any rainier than any other time of year, perhaps even less so. That said, do remember that passing showers are a frequent reality at Walt Disney World, so traveling with a poncho or light rain jacket is always a good idea!
I hope that these thoughts are helpful for you, Christine. Please do let us know if you have any further questions!