Let's put it straight: Children need to know how babies come to the world. Really. They need to know the facts — not storks stories... Different ages require different explanations or details. The explanation for a three-year-old will be different from the one we will give an eight-year-old.
The talk on "How do babies come to the world" is not a one time talk. It's a rolling conversation. Meaning, we need to talk about it when the kids are three years old and again when they are five and again when they are 7 and so on. Each time adding and explaining in accordance with the age of the child, the questions he asks and what is happening around him.
When a child comes with questions, I advise you to answer his questions simply and then wait for the next question.
You can also take advantage of situations around you to talk to your children: If your neighbor is pregnant, if you saw a pregnant woman on a tv show you watched together or if your child's best friend just became a big sister.