Childlore is folklore passed directly between children, without the input of adults.
https://en.wikipedia.org/wiki/Childlore
Stuff I can think of off the top of my head are paper fortune tellers, summoning Bloody Mary/the candyman, the cool S, parody songs like “Jingle bells Batman Smells” and “Joy to the world, the teacher’s dead” etc.
This stuff has intrigued me since, well since I was a kid. I always wondered where they came from. I never saw adults doing that stuff, so I figured another kid had to have come up with it.
This might be local but “Deem!” as the sound of magical transformation. As an adult I think some child heard/read something like “I deem you Sir Galahad, Knight of the Round Table” and mixed it up with cartoon magic sounds, but in our neighborhood any kid with a good stick could wave it and say “Deem! You’re a horse!” or “Deem! You’re a frog!” and the other kid would act the part for awhile. You could even deem yourself, like “Deem! I’m a wizard!” Which is redundant now that I think of it.
Could it be an onomatopoeia of the ringing/shimmering sounds sometimes used? Like an alternate of “ding”?
Yes to that, but also I remember, if I was the fairy or the witch I might say “I’m gonna deem you a princess, okay?” Or even “what do you want me to deem you?” if I wasn’t feeling bossy.