Here's a belief that went way beyond childhood - my grandpa once mentioned to me how odd and unrealistic he finds it when fictional characters dream, since in real life dreams are only memories. Surprised, I informed him that most people's dreams aren't memories at all. He's spent his whole life dreaming only about things that had really happened to him and lived more than 70 years not realizing it wasn't that way for everyone else.
A childhood belief about sleeping

others believed things like this
- I thought the song Shattered Dreams was acutaly "Shadow Dreams" and wondered what shadow dreams were— Golgademon
- In my childhood I used to think that all the superheroes I saw on television existed in real life— Cris
- I used to think that dreams were events that really happened, I just couldn't remember them when I was awake.— Warren
- I used to believe that old people were wrinkly because they spent too much of their life swimming— Amy
- i thought split personalities were like having dreams. the dreams i had were all my personalities!— Odd Dreamer
- I thought Bonnie and Clyde were fictional characters.Anon