I am not to worried about the age a child is taught it by the Parents, as long as the Child knows that they can come to their Parents to ask questions and the Parents are willing and able to sit down and answer the tough questions. I would just prefer it's before they learn it in school.
At least if the kids know it before the schools teach it to them, that should be an easy class for the kids to pass.
Just leaving a book in the kids room sounds like it could work, but that may just start the questions
By the way Letum, that was the book my Neighbor read to me. She was like a Grand Mother to me and my Brother's anyway. I think I was about 8. My father just wanted to joke about it when I was 9 and I walked up to him and asked him about "The Bird's and the Bee's", just to see what he would say. I was very grateful for my neighbor. My father did try to teach me though a while later but his technique just gets into legal issues I won't bring up here. I'll just say, his method did not work for me.