I would recommend you do yourself a favor and subscribe to TourGuideMike. He has the Least Crowded Parks listed for Christmas week, and important strategies for touring during this very crowded period. Shortly, he will update an even more detailed guide for Christmas week. Currently, you can view the 2005 version.
My family has gone to WDW many times over the holiday break, and we will be at WDW over the holiday week this year as well. But trust me, you don't want to be at a park that is at capacity. It is closed for a reason. The park cannot accommodate any more people. It will literally be wall to wall people. Everything from bathrooms to counter service restaurants will be jammed. Lines will be horrendous.
For example, two years ago on NYE at Epcot we were literally in a human traffic jam. It took 45 minutes to get from Mexico to Norway. You could not move either forward or backward. Worse yet, my daughter and I were separated from my wife and other daughter. My kids were crying and wanted to go home. It was VERY scary. This will never happen to us again. From now on, we read and follow TourGuideMike's recommendations. Five trips later, we have rarely waited for more than 20 minutes for any ride.
Some people can survive parks that are at capacity during the NYE and Christmas period and stay in good spirits. All the power to them. But most cannot. For those people, they should listen to TourGuideMike. It IS possible to tour that week and enjoy minimal wait times. But you have to know which park to go to and at what time. HINT: You WILL NOT be going to a park that is at capacity when it is at capacity.
Anyway, best wishes for a wonderful family vacation. If you want to have minimal wait times, you will need to swim "against the flow", or alternatively you will be caught up in the largest human trout stream in history...

Do yourself a favor and subscribe to TourGuideMike.