Angels We Have Heard on High (Just kidding, that is one of the hardest - I love the song but can't stand having to play it 'cause it's tough on just about anything outside of a keyboard.
Silent Night is pretty easy. There were three or four videos of it here this year ranging from my very simple little thing to a couple of pretty nice melody-chord arrangements.
I think O Holy Night is easy if I remember right, we didn't play it this year so my memory might be faulty.
What Child is This (Greensleeves) is a little harder but if you break it down into A and B phrases it's not really that bad (it's basically ABAB through one verse and chorus, and the only difference between the A and B part is in the last measure). Strumming and picking are both pretty easy in Am, and strumming is easy in Dm.
John