Just that really, please can you share with me how old your DC were when you had a discussion with them about sex, relationships, body changes/puberty and everything that goes with it?
Also helpful to know if they prompted it, or was it you as parents? Boy or girl, does this have any impact on the age at which the discussion happened? Did you use a book as a talking point, if so can you recommended one for me?
DS is nearly 9 and has been asking a few questions lately, about my sanpro and the durex lube he came across (hidden under my bed!). He started asking me questions again last night, but was kind of hiding under the pillow saying he didnt want to know about it because it was rude (the sanpro). We had a conversation a couple of weeks ago about no more babies in our family, he asked how did I know for sure and I said we have ways of making sure I don't get pregnant again - he loudly exclaimed oh yeah I know, you're going to eat one of those Durex things aren't you 
so I think we owe it to him to be honest about that, at least!
I don't know what his school's sex education policy is, have no idea if/when it will be mentioned at school, he's coming to the end of Yr4. My friend bought a book (usborne I think) which was suitable for 9+years for her DS, I liked it overall but it seemed to cover way more than I had planned to talk about with my DS 
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DH thinks he knew by the age of about 8yo, some boys he was playing with told him. I had an awareness of periods by about 10yo, but I'm not sure I understood what sex actually was until I was in secondary school at 11yo.
DS and younger DD know that a baby is made by a seed from a man and an egg from a woman, and they know the baby grows in the uterus and then is either born through the vagina or c section (in my case). But they don't know how the egg and seed meet, and they didnt ask at the time she I had my last baby so there was no need for further explanations.