We packed towels, swim diapers, swim trunks and swim shoes for our boys to play in the Donald's Boat water area in MK. However it was about 100 degrees and we planned it to be a refreshing respite from the heat for them. Since you are going in December I wouldn't think your kids will be as interested in the fountains. However it might be worth having a change of clothes just in case. They would probably be uncomfortable and not dry as quickly in cooler weather.
Have a great trip!