In reading some from, Children & Adolescents: Interpretative Essays on Jean Piaget, author David Elkind makes an interesting point about the development in children and their question asking.
"One of the earliest themes in a child's questioning has to do with origins. The beginnings of things, particularly of living things, are a great mystery to children, just as they were to the Greek philosophers."
Elkind goes on to give examples of common questions that children begin to ask around the age of 5 or 6. Questions like "How does the baby get in mommy's stomach?" or "Who was the mother when everybody was a baby?"
Parents should seek to answer these sort of questions in a way that is appropriate to the child's level of understanding.
Elkind goes on to say, "If the parent feels uncomfortable about invoking God in answering such questions he can always ask the child what he thinks and the child is likely himself to supply the theological explanation."
"For since the creation of the world God's invisible qualities - his eternal power and divine nature - have been clearly seen, being understood from what has been made so that men are without excuse." Romans 1:20
Elkind, David. Children & Adolescents: Interpretive essays on Jean Piaget. New York: Oxford University Press, 1970.