When I was around 4 or 5, my mom told me that if I didn't brush my teeth, pill bigs (or roly polies) would enter my mouth at night. She caught me standing in the bathtub brushing my teeth in my sleep after that.
A childhood belief about bathrooms

others believed things like this
- I used to believe that if I didn't brush my teeth at night, ants would crawl into my mouth and eat my teeth.Anon
- I used to believe that eating burnt toast would make your teeth whiter and would brush your teeth for you.— Natasha
- I used to believe when i was about 4 (now 15) that dinosours died because they didn't brush their teeth— gullable
- I used to think that cockroaches would visit your house if you didn't brush your teeth.Anon
- I thought "Angel of the Moring", by Juice Newton said "just brush me teeth before you leave me"!!!Anon
- Until I was about 8 I believed that your back teeth keep growing in until you have a full circle of teeth.Anon