We are going the 18th-23rd. I wonder if it will be a complete madhouse? We have been the 2nd week of December several times and it was never crowded. Unfortunately, DH says I can't pull the kids out of school anymore
We're going the 17th - 23! I understand this week shouldn't be as crowded as the following week, though still more crowded than the second week of Dec.
Advice -- try to stay at a resort with easy access to a park. Also, go to parks right when they open and enjoy the mornings there. Eat breakfast before you go, and eat lunch after you leave. We found it quite do-able to get in most all the attractions we wanted until about 11 am, when the crowds started to become heavier. Don't try to fit in all the rides and attractions you'd otherwise plan to do during less crowded times of the year. Plan to leave the park early and don't go back, because it just gets worse as the day goes on. Once, we made the mistake of trying to leave MK at 1 pm and we could hardly find our way out! And I'd been there 20 times! You can sit on a chaise lounge on the Poly beach and watch the MK fireworks from across the lake, this worked out great for us. Kids can have fun playing in the sand 'til the fireworks start. We skipped the Epcot fireworks but found HS Spectacle of Lights to still be magical and manageable. Spent our afternoons admiring the decorations and gingerbread houses at the different resorts. On New Year's Eve, we hung out at Scat Cat's Lounge at POFQ, much more fun listening to Eliott Dyson's music and banter than being squished in a park.

Because you have children, you'll probably want to think of an alternative way to spend the evening to keep them entertained!
We were told by several CM's that guests staying on-site would still be admitted to the parks, even if they were so crowded that they were "closed." So if you're staying at a Disney resort, don't let those "closed" signs deter you from going to the parks if you don't mind the crowds.
Everyone is right about what kind of clothes to bring -- it can be freezing at WDW in December! Or...not!
That's all the advice I can think of, except to try to stay patient with the increased attendance...because we're all there to soak up the same Disney magic at Christmastime as everyone else!

I will almost certainly be reminding my DH of this several times during our trip!