At that time the disciples came to Jesus and asked, “Who, then, is the greatest in the kingdom of heaven?”
He called a little child to him, and placed the child among them. And he said: “Truly I tell you, unless you change and become like little children, you will never enter the kingdom of heaven. Therefore, whoever takes the lowly position of this child is the greatest in the kingdom of heaven. And whoever welcomes one such child in my name welcomes me."
"Christ points to little children as the model to which the members of his kingdom must assimilate themselves. The special attributes of children which he would recommend are humility, unworldliness, simplicity, teachableness, - the direct contraries of self-seeking, worldliness, distrust, conceit." (http://biblehub.com/matthew/18-3.htm)
Kids are thirsty sponges ready to soak up all you give out. If your output of words is pure, surrounding children are going to absorb those pure words. If your actions are honorable because others praise them as such - in front of children - then kids will drink-in those moments too. Why are they so absorbent, so immersed, so soaked liked sponges at what their environment offers? Because they are experiencing things either for the first time or from a different angle and want to emulate or copy a behavior in their own way. Remember - negative words and actions can be assimilated and emulated too.
The flip side to this coin is that Christ calls us to be like children. He does not ask us to keep all the attributes of a child, no, that would keep us from spiritually maturing. What I believe Jesus asks of the child in each of us is to keep alive the God ordained, useful traits like humility and a teachable spirit - two areas that the Pulpit Commentary lists above which naturally co-relate. The reward for doing so is entrance into and high placement within His kingdom.
The question of rank posed before Christ, by His Disciples, is "Who, then, is the greatest in the kingdom of heaven?" The Lord's response looked past the actual question and into the very troubled hearts that asked it to deliver what they really needed - a perspective and attitude adjustment. Christ answers - first, repent; totally change your thinking about this ("unless you change"). Secondly adopt Godly, childlike traits ("and become like little children"), especially humility ("whoever takes the lowly position") and except for God Himself, you will be the greatest in the kingdom of heaven.
One of the great things about humility is that you get a tag-along - a teachable spirit. If you have one, chances are extremely good you have the other. This is what Christ asks us to be like - a well-mannered child, humble and gracious, sitting attentively in the presence of The Teacher, learning as His Forever Students.