This is about my little cousin who is now 10 years old. When he was about 5 years old (and his mum told us this), he was being very naughty so his mother got angry and said "Don't be so big headed!/have a big head" (and other stuff I suppose!) And my cousin got really upset and sat in the corner and then told his mum "And you've got a big willy!!"
Poor kid, he thought the opposite of head was a willy. And he thought mama had a willy too :)
A childhood belief about rude bits

others believed things like this
- Yeah I believed (because my mother had told me) that your 'willy' would fall off if you played with it :D— Doz
- i used to belive that my willy would eat me at nightAnon
- I used to belive that my willy was a snake and if i wasnt careful it would bite— eggbert
- I used to believe that if i bought a Wonka Bar I would find a golden ticket and get to see Willy Wonka.— Sally
- My cousin's mom told my cousin that pop makes you short. He probably believed it.— Molly
- I used to think a second cousin meant a cousin’s cousin.— Carlos Almonacid