Inventing a fictional character to express both real and exaggerated traits was probably a great source of pleasure to Bellairs, seeing how Childermass could easily have been the author at a later stage in life. Both shared a preferred brand of tobacco, a deep appreciation of history and literature, and an eventual dislike of teaching.
Bellairs kept Childermass true to his established (and arguably predictable) habits and behaviors for the remainder of the books that he authored. Brad Strickland, who completed one book, The Drum, the Doll, and the Zombie (1994), and created three others of his own in this series, said of his first, The Hand of the Necromancer (1996), that he didn't feel he got the character quite right.
"It's the one I'm least satisfied with...too tentative and somehow less Dixon-like than the others. I guess it was that John had no part in it, so it was a scary solo flight. The rhythm of the book feels off to me, and I don't think I did quite as good a job with the Professor Childermass as in Drum, which I confess a strong fondness for because after a lot of effort, I thought I finally got his personality right."
Marcus Childermass, a professor of literature, named his sons for characters in the novels of Tobias Smollett (The Chessmen of Doom, 6). The Adventures of Roderick Random (1748), Smollett's first novel, was an immediate success but made many enemies as most of the unpleasant characters (with names such as Crab, Potion, and Gawkey) were recognized as fellow writers of Smollet.
There is a potential literary influence by way of the 1928 book, The Childermass, by Wyndham Lewis (1882-1957). The novel takes place at the river Styx where a mass hold-up of World War I casualties, including two solders, Pullman and Sattersthwaite, have piled up and are waiting to be processed. Their mediator is a Punch-like character called the Bailiff, who alternately panders to and mocks the candidates with the intent of purifying their motives and helping solidify their personalities. While well-received at the time it was first published - and possibly read by Bellairs at one point - The Childermass is not generally well-known today.
Medieval English tradition may also play a part in naming. Feasts were often referred to as masses, with Christmas being the most obvious example. Childer is the proper and original plural of child and would in turn make the professor's surname children-mass. Historically, the Children's Mass is the commemoration of the massacre of the Holy Innocents - considered by some Christians as the first martyrs for Christ - by the Roman Catholic Church on December 28.
Bellairs may have have remembered the name - but not the date - when creating the character, whose birthday is noted as December 8.