January is our favorite time of the year to go because of the low crowd levels. We have been there later in the month, but I would guess it will be pretty similar. I do know that my school system does not go back until the second week of January, so you may run into some crowds.
A agree with the previous poster about the weather- it is totally unpredictable, but it has been very manageable. We leave very cold behind us, so 50-60* is comfortable for us. The first trip we wore shorts and sweatshirts and sometimes t-shirts. Last year I wore capris or jeans and layered on top. People were swimming, but we spent most of our time in the parks.
The parks close earlier than at other times of the year, but it worked out well for us, because our kids were ready to get to bed. We did not go back for naps, so they were ready for an 8:00 bedtime.
I think you are going to love it! Have a great trip!