The paper bag Christmas / Kevin Alan Milne.

Enhanced descriptions from Syndetics:

Dr. Christopher Ringle is the last person you'd expect to find moonlighting as Santa Claus at the mall on the day after Thanksgiving. But it is there that he meets a young man named Molar Alan, who desperately needs a new perspective on the underlying value of Christmas. Dr. Ringle recruits Mo and his older brother as volunteers at a nearby children's hospital for the holiday season. At the hospital, Mo is tasked to help bring holiday cheer to the young cancer patients on the fifth floor. His biggest challenge is befriending a decidedly angry girl who is so embarrassed by her scarred appearance that she hides her face behind the safety of a paper bag. Almost in spite of himself, Mo finds that Christmas joy emanates from a source far greater than the North Pole, while the young girl learns that she is more beautiful than she had ever imagined.