My last trip was from Jan 4th-22nd/06 and it was awesome. Weather was great, crowds were low and the kids were brave enough to try new rides. I would suggest bringing clothes for all types of weather but not over packing. Being a single dad traveling with 2 boys you'd think I would have only packed 1 pair of shorts and a t-shirt for each of us but instead I packed way too much.
It's tough lugging 3 large suitcases and 2 boys 6,8, with their carryones through an airport, never again!!! When we go back in November, I plan on packing for each of us - 2 Pairs of pants(1 being jeans), 2 pairs of shorts, 2 swim suits, 4 t-shirts, 2 long sleeved shirts, 1 hoody, sandles, socks, underwear, hats, sunglasses and etc...... Now, we will be wearing a light jacket, mitts, a touque and shoes to the airport so no need to pack those but they may come in handy on chilly nights. I found myself doing laundry in the afternoon while the boys were swimming so they only wore 1/3rd of the clothes I brought not to mention all the clothes we bought. Enjoy the weather because no matter what, it'll be warmer than at home. Also check out this website for the weather history for your dates, wunderground.com. It gives averages, highs, lows, precipitation, and wind. It made me feel alot more at ease about booking in January.