Hi Kelly:
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. The week between Christmas and New Years is generally extremely busy as most schools are not in session and families are on vacation. If you have a choice, early January is an excellent time to visit Walt Disney World since schools are generally in session then. Keep in mind that Martin Luther King Jr. Holiday falls in mid-January, and crowds tend to increase slightly during days surrounding the Holiday. Also, the
Disney World Marathon weekend is scheduled for January 5-8, 2012, so you may notice some crowds that weekend. But even with these two January events, I've found that crowds in January are very manageable compared to many other times of the year. So the more you can push your vacation into the new year, the likely less crowded it will be at Disney World.
The weather in Orlando in late December and early 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.