I have been in December at least 20 times. We have had years where we wore shorts most of the time, and other times where it was 40's most of the time. Prepare for anything. I like my son's zip off pants that he had. They were perfect.
Generally, I have worn jeans and t shirt most of the days. In the morning it is pretty chilly and layer with a sweatshirt. Then when it warms up, we put the sweatshirts in a locker until evening. I also keep a pair of black shorts or jean shorts for myself if I get too hot. I have usually only worn them for a few hours a day, so I re-wear them on other days. I always pack swimsuits and I know that my itinerary will change if the weather is nice enough to spend time in the waterpark or pool. Some trips we haven't had them out at all, but I can remember back in 2002, it was going to be 80 one day, so we decided to go to Blizzard Beach for the day. In Jan 2005, there was one day that got warm enough to swim for a couple of hours, so we left the park and we took DS to the pool, and then went into the park later on.
If you are planning on staying late for EMH, you may want to bring a hat and gloves for the kids. I have rode TT and BTMRR when it was in the 40's and I was so glad to have them.