You are going to love being at Walt Disney World over the Christmas holidays - the decorations are beautiful and this is a very special time to visit.
The weather can be very variable in December/January. I can give you the average high/low temperatures for December: 73/53 (with an average rainfall of 2.3 inches), and January: 72/50 (with an average rainfall of 2.4 inches). I was visiting Walt Disney World this past December and the weather was so warm I was wearing short sleeves during the day, and just a lightweight cardigan for the evenings. I also know that the previous year it was cold enough for jackets and scarves. I would take clothes that you can layer, to ensure you will be warm enough if necessary.
If you are considering the Park Hopper option, I think this would be useful for making the most of your time at the parks during this busy season.