My parents told us the basics about sex and reproduction from basically soon as we were old enough to talk and ask the questions. So I don't even remember when I first learned about it because I was so young, but I'm fairly sure it was before the age of 4. It was in very simple terms, eg. the daddy puts his willy in the mummy's vagina and the sperm comes out of his willy into the mummy's body and meets the egg and grows into a baby. We were also told about periods in similar terms at the same time.
I can assure you that I suffered absolutely no trauma whatsoever from having this knowledge... I thought it sounded like a bit of a weird thing to do but then again adults do all kind of weird things that you don't understand the appeal of when you're a kid, like drinking wine which tastes disgusting, or insisting on watching the news which is boring, or reading books with no pictures.
The result was that when I reached junior school and kids inevitably started talking about the odd bits and bobs about sex that they'd overheard from the television or their older siblings, which were often hilariously incorrect, we never believed it for a minute because
Btw, in the 80s, my husband's devoutly Catholic parents gave him a book about human reproduction to read when he was 8 (he knew the words "ovaries" and "uterus" at 8).
You can't control what other children tell your child, or what other parents tell their children. All you can do is