I wonder if it'd really cover anything remotely close to 99% of cases. Even if 100% of parents knew about it and watched their credit cards enough to notice a $1 refunded transaction it just takes something like one friend in high school with a credit card to sign up all their little brother's friends. It may even just cause more credit cards being shared around than kids it stops from getting to the site they want on.

Then there'd be even more unintended consequence. Instead of sites you don't want kids creating accounts on you'd have sites selling 5 minutes of ads to create an account for them or increasingly shady stuff. Preventing this kind of site is the same as the original issue.