Welcome to disABILITIES.
My first suggestion is to look around at the disABILITIES FAQs thread near the top of this board if you haven't already. There are some links to the official WDW Guidebooks for Guests with Disabilities that will be helpful to you. I'm going to break up my reply into sections since it's kind of long.
Location: Any of the monorail resorts would be a good choice. The monorails are all wheelchair accessible so you will be able to get on the monorail easily to get to MK. You can also take the monorail to Epcot, although that does require a change of monorails. There is one that makes a loop from the monorail hotels to MK, including a stop at the Ticket and Tansportation Center(TTC). Another monorail makes a loop between the the TTC and MK. A third monorail makes a loop between the TTC and Epcot.
Besides using the monorail, it is also possible to walk between MK and the Contemporary.
Time of Year: We have been to WDW between Dec 27 and Jan 3 on several trips. The time you are going is one of the busiest times to be at WDW. Literally, you may not be able to spread your arms out without hitting into someone. At that time of year, waits of up to 2 hours in line for attractions are common. Many people assume that people using wheelchairs get to go to the front of the lines so they won't have to wait, but all of the lines at AK and the Studio and many of the lines at MK and Epcot are Mainstream lines. This means that the regular lines are wheelchair accesible and wheelchair users wait in the regular lines with everyone else.
MK is the smallest and most congested park (and also gets the most visitors).
If you are able to go earlier in December or in early January, you will find it very much less crowded and will be able to see more. I realize you might not be able to go at another time because of break being then. Just want to make sure you realize it will be busy and you may not get to see as much.
If you have to go then, some of the advantages are that the parks are open later and there are extra things planned -some special shows and lots and lots of decorations.
Ways to deal with Busy: If you can get up early and go to the parks, that is the best time to go. It's cooler then (may be a bit cold early in the morning in December, but Dec can also get very warm, so it's hard to say). Because the parks are open late, a lot of people plan to get to the park later in the day and stay until closing. If you can get there when the park opens, you have a chance to see things before those people get there.
That may not work for you - it doesn't for us. My DD who has cerebral palsy is at her worst early in the morning. The stiff parts of her are stiffest and the limp parts are limpest, so it's like having an 84 pound rag doll who has a board up the middle of her trunk - loose arms and head hanging when we try to move her and the rest of her is like moving boards.
Some people have used services like Tour Guide Mike (he's a sponsor on the Theme Parks Board) and felt he helped a lot. Other people use touring plans from books like the Unofficial Guide to WDW.
Whatever you do, try to get a feel for the layout of the parks by looking at park maps before you go.
Have in mind a few things you want to accomplish, be flexible and ready to change if it looks like you can't do things when you have planned to do them.
Using Fastpass is a good way to avoid waits in line. If you go late in the day, some of the most popular attractions may not have any fastpasses left, but going early in the day, you should not have that problem.
Things to do: If the parks get too busy, you can easily take the monorail to any of the monorail resorts and look at the Christmas decorations there. They are all grandly decorated (themed appropriately to the individual resort). Grand Floridian has a full size gingerbread house (they sell gingerbread cookies and hot chocolate from the house at certain times of day).
For other things, what is your DD interested in?
Can she get out of her wheelchair (or can you lift her out) fairly easily?
Can she sit on someone's lap/on a seat next to someone or does she need to remain in her wheelchair seating system? Does she have good head control? Any restrictions on what she is able to do?
How tall is she? Some attractions have height requirements.
This is a link to a height requirement calculator. You put in the height of the child and get a list of attractions she is
too small for. That would help you so you know which attractions to not mention to her. For the rides with height requirements, they will ask you how tall she is, but they don't try to measure people in wheelchairs unless they appear to be quite small.
Power Wheelchair: Most of the attractions that are listed as being wheelchair accessible with the wheelchair user being able to stay in their wheelchair are also accessible to power wheelchairs. Some might not be depending on exactly how her chair is shaped and you may not know that until you try to get on.
WDW is a very stimulating and busy place, even when you are there during a non-busy time of the year. It's especially stimulating for children and I have been walked into many times by small children who are so busy looking at
everything that they are not looking where they are going at all.
I don't know if your DD has already been driving a power wheelchair or is just getting hers soon without any experience, but just wanted to say learning to drive it can be a long process. My DD got her first power wheelchair 6 years ago. For the 6 months, she was pretty much terrified to drive it. The way the computer was set up, it accelerated and stopped quickly whenever pressure was put on the joystick. We got that reprogrammed, but she still had to work on muscle control - when she gets excited, she goes into extension. When your hand is on a wheelchair joystick and you go into extension, you go faster and can't stop (since stopping requires letting go of the joystick). That took a lot of work on her part to control, but she is quite a good driver now. She goes all over with little problem (although I still don't walk too close to her if she's really excited). Her driving does suffer when she's really tired (I guess just like mine does if I am driving my van when tired).
No matter how careful your DD is normally driving her wheelchair, WDW is likely going to be harder than any place she has been before. People cut in front of wheelchairs and you may have to stop very suddenly.
Many adults (who are used to driving cars) comment on how intimidating and tiring it is to try to drive an
ECV (motorized
scooter) around WDW with all the things to watch for. If your DD has an extra control (I think it's called an attendent control, but not sure) on her power wheelchair, it would probably be easier and safer for everyone for you to take control while in busy areas. Let her drive at the resort and when you are more out in the open.
Your DD may get tired quicker than usual just from all the stimulation. If she just needs a quick stretch or nap, the First Aid centers in each park are great.
That's all I can think of for now. Ask as many questions as you want and as you add more informtion to this thread, people will have more advice.