Maybe this would more of an "unbelief"...
When i was about 5 or 6, I happened to interrupt my Mom explaining the "facts of life" to my older brother. (My mom was very open about this subject, something I appreciate to this day.) She was saying that when a woman is born, she already has thousands of eggs in her tummy, to make babies when she is married. I looked at my Mom for a while, soaking that in. And then I looked down at my own tummy, and thought "you are crazy; no way are there thousands of eggs in my tummy!" Keep in mind, I'm 5 and I'm thinking chicken eggs...
I just gave my mom that look kids give you: "you are so full of sh--", and walked away...
A childhood belief about reproduction

others believed things like this
- I used to believe that bunny rabbits hatched from eggs, since the Easter Bunny gave you eggs.— goat
- I used to believe that only brown eggs came from chickens. I thought white eggs were made at the store— Destiny
- babies were born from dragon eggs.— Justin
- When I was a kid, I used to believe that cadbury cream eggs real uncooked eggs in the middle Anon
- I used to think that my testicles were eggs that would become babies when I was older...— Martin
- i used to think poo was a worm in me that ate leftovers that my tummy didnt want, that was full and left.— vanessa