it depends on their age and maturity. at 3, from mommy and daddy is usually all they want to know. at around eight telling them that when moms and dads get married (start planting that thought early!) they have a baby. if pressed further, just answer honestly, we have sex. that is usually all they really want to know. as the children get older, they will want more specific answers. remember if you are embarrassed to answer your child, they will become to embarrassed to ask you, and turn to their friends-who generally only know partial truths or guesses. have the talk with your children throughout their lives at age appropriate levels. it makes the big talk so much easier. - also you do not need to tell your kids everything, but tell the truth with what you do tell. if you start lying to the kids because you are uncomfortable, -1. they will not come to you with the important stuff because they will know you lie to them. -2. they learn that lying is ok if you are uncomfortable with a subject matter, and will lie to you about the important stuff in their lives. - remember, i DID NOT say to tell a 6 year old all of the story, but to tell AGE appropriate information.
Answer the question simply and easily. Don't go into detail they will ask you more. The old joke goes like this: The little kid asks his mommy where he came from. His mommy sat him down and told him the whole story and his eyes got bigger and bigger. When she was finished the little kid jumps up and says, "Boy! Wait until I tell Tommy! He says he came from Cleveland!"
I get out the books and have a scientific talk with them... the depth of the process depends on the age of the child... no storks or cabbage patch, belly talk implies you don't trust their intelligence... if they are old enough to ask they are old enough to hear some "depth" of the truth.
The very first thing you say is " that's a good question, please tell me where YOU think they come from and ill be glad to help you understand what you already know. At least then you will have an idea where they are in knowledge and you can say you are right, or correct them more easily.
Depending on the child's age, I would tell them a baby comes when a mommy and daddy love each other so much, a stork puts a baby inside mommys tummy and she carries the baby around for 9 months until the baby wants to come out. :)