Hi Karen:
What a great treat to your family to take them to Disney World. The basic rule about crowds is that they tend to be heavier when schools are not in session. January is an excellent time to visit Walt Disney World since schools are generally in session then. I'm not sure what days you coming in January, but Martin Luther King Jr. Holiday falls in January, and crowds tend to increase slightly during days surrounding the Holiday. Walt Disney World
Marathon Weekend also falls in January, bringing in runners, friends and family. However, even with these two January events, I've found that crowds in January are very manageable compared to many other times of the year.
The weather in Orlando in January usually hovers around 70 degrees fahrenheit. However, during certain cold snaps, the weather can really dip low, to the 40's or even lower. I was at Disney World two years ago during the first week of January when it was unseasonably cold. However, when I was at Disney World three years ago in early January, the weather was in the normal 60s and 70s. So it's always a good idea to dress in layers in case the weather changes unexpectedly. Tee shirts and shorts are likely fine in the day, but long sleeves and long pants may be needed at night. Precipitation is relatively minimal in January as well, but since it's Florida, it can always rain, seemingly at any time.
Enjoy your vacation.