We are going to be at the resort during christmas week...
We've gone during Christmas week- twice. Probably need our heads examined, but it
can be a magical time to go. Buses are the least of your worries. Parks, especially MK, can fill up and close pretty early in the day. Around 10 AM for the MK on one of our days, if I recall correctly.
I'd suggest going early in the day for a few hours and then go back in the evening. After the fireworks, the crowds will be very manageable and the parks will be open much later. Some rides like Kali or Splash Mountain can be walk-ons since many people (except some, like us crazy Canuks) think it's too cold for water rides. Come on, really, it's still liquid, what's the problem, lol!
Your fellow park-goers will be the real challenge. On one of our trips we arrived on Dec 18th. Everyone seemed pretty cheerful and just glad to be there, nice, but still crowded. Then on the 22nd or 23rd, things changed, it wasn't just crowded but a tone of "desperation" seemed to settle in. We saw way more kids melting down, like a girl around 12 years old bursting into tears because her drink was gone. And we began hearing a lot of the "this vacation cost a lot of money so you'd better..." talk. It was like everyone who had driven across the country nonstop, went straight to the parks without any rest that day. I actually felt sorry for quite a few of them, as I've certainly experienced the parks on little sleep after flying all night to get there. I can imagine them going on this "magical" vacation without knowing about the research needed ahead of time and then, wham! Full park mayhem without any warning.
Now, after all that, I wish you a wonderful trip. Disney is great at Christmas but do manage your expectations.