Honestly, William as Prince Charming defending his pathetic little princess from atop his white steed romanticizing aside, I doubt the Middletons are invited to Sandringham for Christmas day. They were invited to shoot on Boxing Day last year. That may be the case this year as well. But we should all know by now that the sources reporting this are not particularly reliable. If we believed US Weekly, Kate has been pregnant about 15 times prior to this pregnancy, after all.
Also, quite honestly, I think that Kate, being a 30 year old woman who is educated and was quite independent prior to marriage, can probably handle Christmas day without her mommy and daddy just fine. She probably doesn't even need William riding around on that horse to protect her.