When I was little I thought I had two bums because my mom referred to my vagina as my "front bum" and my actual bum to my "back bum."
Also, I thought that guys wore panties just like girls-I never heard of boxer shorts until grade 2 or 3 when my friends laughed at me for referring to a boy's underwear as his "panties" (they had little brothers which is probably why they knew the difference.)
A childhood belief about rude bits

others believed things like this
- I used to think my brothers penis was a hotdog, until i put one in my panties and the dog ate it.— Amanda
- I thought that the difference between girls and boys was that girls have teeth.
Anon - when I was a kid, I used to believe that only girls could be my friends and all other boys were my brothers.
— rajat - I don't know why or where I got this from, but when I was little, I referred to my vagina as my "goose". — StrangeLilGrrl
- when i was a little girl, i beleived that women didnt have bums, because i never heard my mom fart. Anon
- I once believed that if I wore my Halloween costume to bed, I'd wake up as the actual thing.— Sarah