M is for Mary Poppins syndrome. Remember, each time the children have had wonderful adventures, Mary Poppins promptly denies that anything out of the ordinary has happened, but there is always a tiny detail confirming that it has. Thus every time the narrative modality is ambiguous, the text may give a prompt to support our interpretation in either direction. I have not seen this term used by anyone else.
M is also for magic, which I have written extensively on in most of my work.
It is also for metafiction of which I first wrote in Swedish in 1991, and then in a chapter in Children's Literature Comes of Age. Today every first-year undergraduate knows what metafiction is, but I am so old that it was new then. In fact, the term metafiction was coined at a conference in the early '70s, although the phenomenon had of course existed long before. You might not associate metafiction with early children's literature, but if you re-read Andersen's Thumbelina, that's exactly what he does in the end, not to mention MacDonald's At the Back of the North Wind. So if your children's literature instructor tells you that metafiction was invented by David Wiesner, you can interrogate it.
Mary Poppins syndrome is a metafictional device.
M is also for memory which is a significant aspect of fiction that cognitive criticism deals with.
The Dormouse would suggest that M is for muchness, but I am not good at drawing.