Matthew 18:3 and said, "Amen I say to you, unless you turn and become like children, you will not enter the kingdom of heaven."
As children are completely dependent on, and trust in, their parents, Jesus also wants the disciples and us to be dependent on God.
Matthew 18:6 Whoever causes one of these little ones to sin, it would better for him to have a great millstone hung around his neck and to be drowned in the depths of the sea.
Here Jesus may be referring to those who are little in their faith. Whether it be a new believer or someone who is struggling with their faith, or something in their life like an addiction. Whoever leads them astray will not be tolerated by God. It is our job to guide those people back to God, not to help them continue to fall away. You wouldn't give an alcoholic a beer or someone addicted to porn a magazine of naked pictures.
Just as our children trust in us, we must also trust in God. And we must show the "little ones" how trusting in God is better than the alternative. I don't always feel worthy of the task because there are days I feel like I am falling. But I trust in Jesus, and I trust he will catch me, turn me around, and welcome me back with open arms.