This doesn't sound like a "cancel the trip" kind of situation to me. If she's still going to school (where there are crowds and jostling because...kids), sitting in a hard desk chair all day, carrying a heavy backpack up and down stairs, etc, and the doctor wants her up and active, then it sounds like she can handle walking around MK during the Christmas party and probably even riding some gentle rides. Sure, they can stop/start abruptly, but so can cars and buses on occasion, and I'm assuming she is cleared to ride in/on those.
"Jingle" Cruise, Carousel of Progress, Laugh Floor, Country Bears, Tiki Room, Philharmagic, Small World, Peter Pan, Haunted Mansion, all seem to me like rides someone who can carry a heavy backpack up stairs should be able to do okay with. Pirates might be a little risky with the little drop/plunge, so I'd avoid that. And obviously none of the Mountains or Mine Train, or anything that spins and twirls. I'd ask the doctor what he thinks just to be sure, but I can't see why, given what your daughter can do at school, she couldn't ride the gentle dark rides.
Honestly, there's so much more to do at the Christmas Party, especially since you've never been to one (I haven't either, going in a few weeks!), that you guys might find you don't even want to take time for rides. Meet the characters, watch the shows, parade, and fireworks, shop, eat cookies, and just enjoy the atmosphere. Like you say, it'll be a nice pick-me-up for her.