I was about her age or younger when I first remember having that kind of talk with my Mum. Just small stuff because I'd hear things from bigger kids, I'd ask what things meant and Mum or Dad would answer in simple terms with no fuss.

I remember watching Hocus Pocus and they were laughing at the older brother for still being a virgin. I asked what a virgin was and Mum just said "someone who hasn't had sex." Another time I heard about condoms (no idea where) so I asked Dad what they are and he just said "something a man wears when he doesn't want a baby." They were simple answers that I was happy with.

Wroth regards to actual sex, I'd steer away from it being an adult word - I think it's ok for young kids to say the word, you just need to tell them it's not necessarily appropriate to talk about with their friends at school. I'd tell her that even though the word isn't "adult" and she can talk to you about it whenever she needs/likes, it means something that adults do and therefore shouldn't be talked about with other kids. At her age, telling her it's what adults do to have a baby might be enough for her. You can decide later how you want to approach it - something adults do when they love each other/because it feels good/etc.