Any time I have heard/read anything from a "professional", with regards to talking to your kids about where babies come from, they always say... keep it simple.
When Paige was 5 she began asking questions. I kept it simple. I only answered questions with a direct response... no elaboration. My simple answers were never good enough for her. All it caused her to do was ask more specific questions. By the time our conversation was done, we had pretty much covered the curriculum in high school health class.
Thankfully I am not, in the least bit, shy about discussing the topic.
But, hubby is not so fortunate. He would have rather that I told her, "It's all magic."
Paige quickly caught on to this and had tons of fun chasing him around the house yelling the word "uterus".
Jump ahead almost 4 years.
Over the weekend Paige and Daddy were cuddling on the couch. I sat down to watch TV and Paige decided that it was my turn to get some cuddles. Daddy started teasing her and made a pouty face because she had left him.
"It's Mommy's turn."
"What about my turn Paige? What am I, chopped liver?"
"No Daddy but Mommy gave birth to me. You just gave the sperm."
He magically disappeared.