we were at WDW 2nd week December. The crowds were relatively light and the weather could not have been nicer. It was about 78-82, sunny and dry everyday except our last. On our last day we got up, dressed everyone in shorts and t shirts and walked out of our room to find the temps had dropped to about 50 and WINDY overnight. We drove to MK and put on all of our warm clothes in the van and still had a great time. The weather can vary so greatly in mid FL in the winter. Check the 10 day forecast and pack accordingly. I saw plenty of children that day freezing their you-know-whats off. PS...If you can avoid the MK on Saturday. It is always crowded, no matter the time of year.
Good Luck!