Children would not exist without adult genitalia.
I see no reason to go out of one's way to show adult genitalia to children, agreed,
but an accidental glimpse should hardly be traumatic.
Prudish and prissy Americans create forbidden fruit and perversity with their own hangups.
A classis example is the 21-year-old drinking age.
Does it make any sense whatsoever for someone to begin drinking small quantities of alcohol for the first time in adulthood?
Is this why so many college freshman
inflict serious alcohol poisoning on themselves
as soon as they're outside of their helicopter parents' view?
Sophisticated families introduce a simple glass wine at dinner to kids in their early to mid teens.
This takes away the forbidden fruit quality of adult beverages which causes kids to abuse them badly when unsupervised.
It also teaches them one small part of being a civilized adult.