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.
Man oh man I could probably have a whole thread just for video game urban legends from the late 90s. That’s when some of your friends had internet, but maybe you didn’t, so there was just enough legit info to make the fake stuff seem believable (looking at you, MissingNo). Luigi in SM64, all sorts of secret Smash 64 characters, literally the entirety of Pokemon.