We have been at Christmas twice...here are my tips and suggestions for making it a great trip.
Since you are going at a time that
MVMCP will be going on and about to end, I suggest you don't pay the extra money. It ended this year on the 17th - we went, and then on the 18th everything they had offered "special" was done during regular hours. You will be able to see the same shows, parade, decorations, etc. for free the week of Christmas.
The crowds will get busier and busier as you get closer to Christmas. When we were there on Christmas Day, we went to MGM instead of trying MK. It was pretty empty most of the day there....definitely avoid MK on this day, as it closes down full to capacity.
Depending on the ages of your children, etc, they offer a wonderful tour called the Yuletide Fantasy Tour which we did this past Dec. They take you around to Epcot, MGM, MK and the Christmas Warehouse showing you decorations and how things are done. Your children have to be 16, but it's really an awesome tour.
While at Epcot, try to see as many of the Santas as you can...they are really neat to hear how the holidays are celebrated in other countries --- don't miss Norway's gnome, he's adorable.
MGM's Osbourne Spectacle of Lights is really fun and it "snows" on you while you walk around looking. Don't go immediately when they turn on the lights or you might get trampled!
DON'T MISS the Candlelight Processional! It's at Epcot and they have guest readers come and read the Christmas story from the Bible. In between verses, they have a choir which sings Christmas songs. It's awesome, but busy. Sometimes there are long lines just for stand-by, so I strongly recommend the dinner package. Then you just walk in.
Illuminations this year did a special holiday ending that was the most amazing fireworks spectacle you will ever see...make sure to stick around for this after a day at Epcot if you can.
Check out the big hotels. The major hotels do major decorations! They all have huge trees, and then some. The Grand Floridian has a life-size gingerbread house (real gingerbread) that you can buy gingerbread from....delicious gingerbread IMO. The Boardwalk does creates a candy Boardwalk Amusement Park. AK Lodge creates an edible African marketplace, and I believe Beach Club has the life-size, rotating gingerbread and candy carousel. Amazing creations....sitting there waiting for you to come and see!
Hope some of this info. helps! Christmas is a wonderful time to go....just be prepared for the crowds and you will have no problems. Have a great trip!