I've heard it reported that if you go early, like at opening time, on Christmas day that the parks are empty. I guess lots of folks are either sleeping in, doing the Santa thing, or having a big breakfast. If it were me, I'd try to get a late breakfast PS (10-11am) at 1900 Park Fare at the Grand Floridian, hit Magic Kingdom at opening for some fun, then head on over to breakfast at the Grand.