Hello there Jo!
Oh, I wish I could be there in January as well!!! Going to the water parks in January really all depends on the day and how much tolerance you have for chilly weather. The pools at the resorts and water parks are heated, so water temperatures are fine, but once you get out of the water, you may get a little cool.
Of the two parks, my favorite rides are at
Blizzard Beach! But, my favorite overall theme and park has to be
Typhoon Lagoon. I just love the Lagoon's massive Wave Pool! You won't have to worry about choosing the park though, because at that time of the year, only one park will be open due to refurbishment. And that water park will be, drum roll please.... Blizzard Beach!
You and the kids should have a great time at Blizzard Beach! Make sure to go on Summit Plummet and Teamboat Springs, two of my favorite water rides anywhere! I hope this information helps you with your planning and if you have more questions, please ask away, we are always here to help! Thanks so much for asking the Moms Panel!