I like I am going to be the contrarian of this post, but here I go!
I think if you are used to coming to WDW during spring break and summer time, I bet the week before Christmas next year will not be any more crowded than what you have experienced. Probably less crowded on the Mon, Tues, and Wed of that week. (I assume you meant the week of Dec. 15-22.) I'm betting the way Christmas falls on a Tuesday, most school systems will still be in school for the beginning of that week. Our school calendar (Florida) just came out for next year. We get two full weeks off and the vacation will start on Thursday, Dec. 20th - Wed. Jan 2nd. I know my niece and nephew that live in the NE typically get less time off than we do, so I expect them to have Dec. 24 - Jan 1st off.
You would probably love coming here during Christmas time. There are a lot of special activities and decorations that are a lot of fun. I think with good planning, you would be able to experience most of them. I love the Osborne family light display in Hollywood Studios and have gone every year since 1998. The Candlelight Processional is a traditional telling of the (religious) Christmas story in a cantata format at EPCOT. My husband loves that. Epcot also has a lot of little traditional holiday activities and displays from around the world in the countries. The castle looks absolutely magical lit up in its sparkly clear lights.
My tips:
If you go, make reservations for dining! Find out about special dining programs. For instance, they have a special package for the Candlelight Processional. You make reservations for dining at a certain time and then you get admitted into a special reserved area of the pavilion for the Processional. The cost is about the same as going to the restaurant and ordering off the menu, but you have guaranteed seating. (You still wait in line, but they let those people in first before the line of people just waiting for the show.) I did this a year or two ago on either Dec. 23rd or 24th coming back from a cruise with a group of 6 and had great seats even though the park was crowded. They also have a similar thing with dinner and reserved seats at Fantasmic in Hollywood Studios and Finding Nemo in Animal Kingdom. (Ask about using Tables in Wonderland or Dining Plans with these programs. Not sure if they allow it.) I would definitely recommend doing this dining/reserved seat thing on the Thurs., Fri and Sat as it will probably be busier those days. (I personally make a sit down reservation almost every time we are in Disney. I think it's nice to get to get out of the crowds, take my time and relax for a little bit to recharge. And I don't mind taking my time and getting my money's worth on refills of Diet Coke!)
When you go to to the park, have each person choose one ride that is their top choice. You can get a fast pass every 2 hours. I don't know how old your kids are, or what they like, but there are certain rides that run out of FP's earlier than others. I try to get those first, depending on our family's priority. There are times for FP return, but you don't have to stay in that window. You can use them any time after the start time up until the park closes, so don't worry about running from one side of the park to the other. (A lot of people don't know that they will take the FP after the time. I have found discarded passes left places on the FP dispensers. I take them and silently thank the uninformed!

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We live pretty close to Disney and have annual passes, so I am probably more relaxed about seeing and doing everything than some of the posters. I have been during almost all times of the year though. I never get to the park when it opens. When we go during busy times, I tend to go around 4 pm and stay until closing, taking a break for dinner. The parks stay open later. Magic Kingdom is usually open until 11pm or midnight. I have been there as late as 2 or 3 AM on Extra Magic Hour days.) I find the crowds thin out after the parades and fire works. My kids are older and are also night people. The people that tend to stay are adults or teens, so some of the "little kid" rides hardly have a wait. The big rides are much less crowded as well, in my experience.