The first sex education lesson I had was in 6th form. All I can remember is the lady rolling a condom on to a banana!
I spoke to both my children. I bought a book to help me explain. I think it was by Claire Raynor. The illustrations were fairly explicit. I pitched things as and when I felt the children were ready. One day my son asked how babies were made. Just as I was about to launch into an explanation...deep breath....he announced he knew, rushed to get the book and said, " Look, the man lies on top of the lady." Then he looked at the next page, which showed an erect penis and said, " that's bigger than Daddy's!"
I think sex education is ideally best done at home, where the parent can pitch it at the level and understanding of the child. But as many of us know it is often avoided, maybe because of embarrassment. Schools have to provide this service, but sometimes the child may not be ready. I was working with a 10 year old girl, who was quite distressed after someone had come in to school to give lessons. She told me it was disgusting and couldn't imagine doing anything of the kind when she was grownup. She simply wasn't ready.
Just as important and from a very early age, I told my children not be afraid to say no to anything that made them feel uncomfortable and that they should never feel compelled to do things because of peer pressure, bullying etc. Also, if they were touched or spoken to in a way by someone, that felt uncomfortable, then it was ok to tell either us or a trusted adult, regardless of what the person said.