This isn't so much childhood belief as childhood logic.
When I was a kid and I was dreaming I was always terrified at the fact that I couldn't figure out whether I was dreaming or not. But one day I realised that whatever you don't want to happen in dreams is generally what happens. So I took a fear I used to have as a child of being scared of gorillas to overcome this.
When ever I was sleeping and not quite sure whether I was or not I would use reverse psycology on my subconscious mind and repeat the words "I hope a gorilla doesn't show up" over and over, and if one appeared I realised I was in a dream and had nothing to be worried about.
A childhood belief about sleeping

others believed things like this
- when i was a kid, i used to believe i was actually a king who was in a coma dreaming about being a kid.— r
- I used to belief sloths lived in trees outside my childhood home. My older sister told me so.Anon
- Yesterday I realised that invisible ink doesn`t make me invisible.— Jonathan Eleven years old
- I was convinced that Gorilla soldiers in Vietnam in the 70's were real Gorillas attacking other Soldiers.— Mario
- that Guerilla Warfare was called "Gorilla Warfare" and it's about fighting gorillasAnon
- In childhood, I waited behind the theatre, after the Cinema, hoping to see the artists came out .
— mprnair.