You can also read sex talk with a 6-year-old. This article gets a little tricky. The author suggests saying something basic and seeing what questions the child has. She says most kids know less about sex than we think. They just get bits and pieces, so it's important for parents to explain it in a scientific way.
This advice is from the site talking with kids:
"Talk about sex in a way that fits the age and stage of your child. If your 8-year-old asks why boys and girls change so much physically as they grow, you can say something like, "The body has special chemicals called hormones that tell it whether to become a boy or a girl. A boy has a penis and testicles, and when he grows older his voice gets lower and he gets more hair on his body. A girl has a vulva and vagina, and when she gets older she grows breasts and her hips grow rounder."
Wow, I didn't realize how much I am not looking forward to this. I can see my son looking back at me like I'm crazy already. When I explain things to him now he says, "Mom, you have never told me that before" in a tone that means, "I can't believe you waited this long to tell me."