I highly recommend the book "Where did I come from?" for this kind of situation. I remember my mom got that book when my sister and I started asking the tough questions. She was very forthright, read us this book and answered all questions. Sometimes we got the answers and other times the answers clicked as we got older. But, even if we didn't totally get her answers, we knew that we could ask any question and get a straight answer. I've always appreciated that about my mom.
Fast forward to when my almost 17 year old was in 7th grade. He started bringing home the most insane questions about sex. Of course, we had had "the talk" and read "Where did I come from?" so I knew he knew the basics. So, in honor of my mom, I just started answering questions in a forthright, clinical manner first, then re-answering them from an ethics point of view. After a full week of this, to and from school, I asked what was going on. It turned out that after the first day, EVERYONE in school learned that "Will's mom" would answer ANYTHING and they brought him all their questions. Priceless.
So, for me, the moral of the story was, either I can tell my kid what I want him to know OR he's going to hear it "on the streets." I'd rather it be me.
This story has far more impact for me today, as I learned yesterday that one of my most favorite students is now pregnant and, according to my son, a 14 year old freshman just had a baby. I'm sorry to say, I don't think 8 years old is "too soon" anymore.
<sigh>