Mat 18:3-6 and said, Verily I say unto you, Except ye turn, and become as ittle children, ye shall in no wise enter into the kingdom of heaven. hosoever therefore shall humble himself as this little child, the same is the greatest in the kingdom of heaven. And whoso shall receive one such little child in my name receiveth me: But whoso shall cause one of these little ones that believe on me to stumble, it is profitable for him that a great millstone should be hanged about his neck, and that he should be sunk in the depth of the sea.

I was considering this in the vein of being converted (obeying the gospel) and becoming "children of God"? We can be child-like or even childish and still not be a child of God.
This just may be setting the mind-set for obeying the gospel - having eyes that see and ears that hear, but not understanding - since He told things in parables in order that only the disciples would be understanding at this point in time.

Understanding that the disciples were thinking in terms of the present (who IS the greatest?) as was seemingly the only way that they perceived thoughts and events, He had just finished showing them how to deal with taxes, an act concerning the world in which we live, and then reminding them to give to God what is God's. We can deal with the world in a child-like or humble way, but still give to God what belongs to God.

He followed that with the admonition of not causing a "child of God" - these little ones - to stumble, for we are to receive such in His name, and in this manner we receive Him.