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.
You can make yourself wake up at 6am tomorrow by hitting yourself on the head 6 times, you need to do it really hard though.