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.
Going way way back to the 1980s… there was a rumour around my school that there was a secret chocolate factory hidden in Super Mario Bros 1. We didn’t really think there would be much to it, but we’d all been to the minus world and we saw the palette swap in some worlds (like I think world 6). The more reasonable or computer/code minded of us just figured it was a glitch that palette swapped everything to browns, so “chocolate factory” became the name for it, where it didn’t officially have one.
Nintendo must have heard this rumour, because Super Mario World featured a “Chocolate Island.” But this was before SMB2 was ever a thing.
I’ve heard of this (way after the fact). IIRC it was part of some player’s guide, and may have been included either as a joke by the author or a copyright trap by the publisher.
Seems I was right. Per this wiki article Apologies for linking to Fandom.