She was ecstatic when we ended up with a rooster in our ‘sexed’ chickens even though Mummy and Daddy weren’t too impressed, because she wants more little fluffy chicks. I think that counts as evidence she knows you need boys and girls to get babies.
…
“What would happen if there were no boys?” This is the question she came out with the other day.

“Well, there would be no babies.” I didn’t quite get the problem seeing we were feeding the rather prolific guinea pigs at the time and we’ve been talking about separating them.
“But what if there were no boys at all? So no-one could have babies?”
“Eventually, they would all die.”
“But there wouldn’t be any left!”
“No, they would die out.”
…
“How did humans start?”… a puzzled little crease in her forehead. “Because if there weren’t any around with the dinosaurs they must have started sometime. How could there be no humans and then they’re there? What was the Mummy?”
…
“Well, they weren’t sudden. You know how you are a little bit different to Mummy and Daddy? That’s how it happened. The babies were just a little bit different to their parents and over a really, really long time they became people.”
…
“But if there was only one it would die out.”
Well that came out of nowhere, did I miss half of this conversation?
“You know, the first person. It needs to have both a boy and a girl to have more people babies. So if there was only one it would die out.”
“Oh, from yesterday. Yes, you’re right. But people live in groups, so they’re all changing a tiny little bit at the same time.”