This NPR article discusses some of the things people used to believe as kids. Such as people thinking sea horses were make-believe, unicorns were real, or "when my dad had the graveyard shift he literally had to work in a graveyard."
We have these signs in California, prohibiting large vehicles from traveling down certain streets:

When I was a kid, I thought this meant the streets were only a few inches thick, and if a heavy truck drove on them, they would crumble.
Also until pretty recently, I thought Warren Buffett got his start by investing the royalties from his song "Margaritaville."
So what silly things did you once believe?