Just keep in mind that the holiday week between Christmas and New Year's is the absolute busiest at WDW.
Especially since you're staying offsite, you're going to need to get up early each morning and get to a park before they fill up and close to outside guests. Also, plan your ADR's to match which park you plan to go to each day, since you probably won't be able to parkhop on the busiest days (if you leave a park, you may not get back in, and you may not get into another park either). Avoid EMH days since you can't take advantage of them...those parks are usually the most crowded each day, so plan to go to one of the other parks instead.
It's beautiful, and very magical...but very, very crowded.

We were at Epcot for the first part of the day on New Year's Eve a few years ago. You LITERALLY couldn't move. It was wall-to-wall people...worse than "Toronto subway at rush hour" crowded. We had to hold hands so we wouldn't get separated and lost in the crush. Luckily, since we were staying onsite we could still parkhop (they have different criteria for closing to onsite vs. offsite guests during peak times) so we went over to MK which was a bit better. It was still the most crowded I have ever seen it.
I'm not trying to scare you, but just to prepare you for a bit more of a hectic time than travelling to WDW at some other time of the year. If you plan ahead and prepare everyone, it should be fine...but I wouldn't want to arrive there and be surprised by how busy it is.