It really depends on what your family considers cold. The week befor Christmas last year I though most days a little cool, and I am never cold, but there were plenty of people in the pools.
we were there from 12/29 - 1/3 this past year and I would say it would be fine at the water parks! Highs were in the mid 70s to low 80s most days. We took our kids to the pool several nights after we were done with the parks. The resort and waterpark pools are heated so I would bet you'd be fine, especially going during the day.
We ahve been to WDW 3 times since 97 in that time period and it really depends. Mother Nature sent us 40 - 50 degree weather that first trip and a mix on the 2nd and this last one, like another poster said it was 70 - 80. You just never know.