I have to share this story. It is one of my favourite Mighty family tales.
When The Boy (my 13 year old) was about 5 or 6, he was also very knowledgeable about the human body. We had a terrific Usborne book that taught all about your body and all it's systems including reproduction. I had bought it as a book for us to look at together. But my boy was a super-reader, and even at 5 and 6 years old he would read anything and everything he could get his hands on. Including this body book. Needless to say, he was quite knowledgeable in how it all works. We didn't realize quite how knowledgeable until one day while we were driving in the car.
We had a neighbour girlfriend with us, who was also 6 and rather precocious. They were having a chat about when they would be grown-ups.
The Boy, "So when we get big, we will have to get married and have lots of kids."
Girlfriend, "I don't know if I want to have kids when I grow up."
The Boy, "Oh don't worry, you will. I am going to put my sperm in you."
After Daddy and I stopped choking, sputtering and laughing, we directed the conversation elsewhere. Then we tried to figure out how to break it to Girlfriend's parents that our little boy had been giving their daughter some sex education.
Thankfully her parents were totally cool about it and were as open with their kids about that kind of stuff as we were.