This isn't actually my belief, it is (or was) the belief of a classmate of mine. She thought that babies were delivered by a stork. Sort of reasonable, right? Wrong. She wasn't 3 or 4. She was 15. And in Honors Biology.
A childhood belief about reproduction

others believed things like this
- Until i was 7,I believed that the stork actually delivered babies.
I was a very stupid kid....— DumbKid - I used to belief that women had babies by pressing their belly buttons!— amy
- I used to belief that babies came from babies r us.— Honey
- ı used to belief I thought astronauts could see GodAnon
- I used to belief that if you stood behind the TV, you would appear on the screen.— Gen Rodenis
- I used to belief that if you bought a car you needed to put it togetter yourself— Bugs