When my mom was pregnant with my sister (I was about 3 1/2 or 4) I noticed that her normally innie belly button got flatter and more shallow as her belly grew. I assumed this was the purpose of a belly button- it was just twisted up extra skin that would unfurl to make room when the woman's belly grew when she was pregnant. I assumed men had them for the same reason they had nipples- decoration.
A childhood belief about belly buttons

others believed things like this
- I used to think that if i touched my belly button it would unfold and my belly would be inside out forever. Anon
- My mom says that when she was growing up, she thought that her belly button was where her skin came together.— Dana
- I used to believe that if your belly button came undone your skin would fall off! Anon
- My younger sister used to believe that in everybody's belly button there lived a family of squirrels.— Sophie
- babies come out of your belly button— Kit
- when I was a kid I thought that babies come out of a belly-button.— Ana