I personally don’t like the idea of telling 3/4 year olds knowing what sex is a penis going in vagina etc
You don't necessarily need to tell a 3 or 4 year old this - Daddy puts a seed in Mammy's tummy might be fine.
But if they then ask 'how?', you need to follow up with the answer, and context.
Regarding kids' shows, by the time they are 8, a lot of shows make reference to kissing, relationships, having babies. J remember some references in Clifford the Big Red Dog that we saw at Christmas!
My kids tended to ask me, so I answered.
Also, I think it's vital from early ages, children then teens, understand about consent, treating people respectfully, respecting sex instead of seeing it as the subject of a crude joke. I want all my DC, boys and girls, to be able to speak openly about sex & understand the issues for the opposite sex.