I used to think that girls didn't have a word for their private parts, because I had always heard the word "penis" (I have a little brother) but my mom always referred to my sister's and mine as simply "private parts." Then I thought they were called "chinas" because we picked that up somewhere (obviously mishearing it). Good case in point to be upfront with your kids.
A childhood belief about rude bits

others believed things like this
- When I was a child, I used to believe that armpits were private parts— Duda Gueiros
- When I was little I thought girls had "man parts" as well, but it just took longer for them to grow.— person
- When I was younger I was certain my "private" was called a ba-china... the boy's were pickles. — taliba
- When I was a kid, I wondered why they called it "public hair" when it was so private.Anon
- In first grade, I thought that if I've never heard of a word before, then it wasn't a word.— Destiny
- i used to think that the word "stupid" was a curse word, ranging in scale right beside the "f" word.Anon