Suppose we were asked, "Who are the greatest people in your town?" What would we turn to as qualities that exemplify greatness? Would it be money, prestige, learning, or military conquest? Just before this incident, the disciples were arguing about which of them would be greatest among them (Mark 9:33-37). Jesus responds to the disciple's question by using a child.
He is not saying that heaven is populated by little children. He means that a child's attitudes of easy dependence, trust, unpretentiousness, awareness of weakness, lack of knowledge, and submission to parents illustrate what we as converted adults must become toward God. Jesus is not saying every child is like His illustration; He is using one to illustrate an ideal. It is essential that we grasp that we must turn, change, to become like the ideal.
John W. Ritenbaugh
Fully Accepting God's Sovereignty, Part Three: The Fruits