I had to explain to my younger brother and sister 10 & 7 about sex. With my mums permission of course, because the extent of my "talk" of anything remotley to do with a womans body was mum asking me if i knew what a period was, i said yes and that was that. I first asked them casually if they knew how babies were made. They both said no, so i explained it to them, with diagrams of a womans uterus and cervix. The diagrams really helped them understand where the egg and sperm met, how the sperm got there, where the sperm comes from. It really opened up a dialouge with them, they ask me all sorts of questions now lol even about ivf! I used penis and vagina, my brother did call his penis a d***, we corrected that. I really tried to make them feel comfortable and not intimidated at all.