A friend of mine passed on the link to this story: The Queen's Return
Actually it is far more than a Christmas story. It is a great example of the genre of fan fiction, and is a must read for any Narnia fans, as it poses an answer to the question of what happened to Susan.
I read it out loud to the kiddoes yesterday, and they really liked it. It was a bit of a tearjerker, at least for me. Great story. Captures the essence of C.S. Lewis.