I used to lie perfectly still when trying to get to sleep when I was about five. This was because I believed that all of the creatures that lived in my matress would feed off any dead skin that I managed to slough off in the night.
Alas, the belief is actually true - my parents let me watch a TV programme about dust mites, which do live off shed skin... but had merely failed to point out that they aren't huge... it was just that the pictures were taken with an electron microscope. So actually, I suppose I believed that dust mites were the size of, say, a comon domestic cat.
A childhood belief about under my bed

others believed things like this
- My mom called the dust under my bed kitties i believed there was kitties living under my bed.— Kathy
- In 'Call Me Maybe', I misheard 'Ripped jeans skin' as 'Rich cheese skin'. — Carly Rae Cheddar
- When I was 6 or 7, I believed that people could have yellow skin like in the simpsons— Noah
- I used to believe that the dust on top of doors was poisonous.— draconid
- What about "Another one buys a Datsun." You know, instead of "Another one bites the dust.— e-man
- I used to believe that the song "Another one bites the dust" was "Another one rides the bus"— Emmy