Matthew 18:1-5, 10
It's very human to try to make the Kingdom of God fit into our paradigm. Leaders -- the disciples in their time, just like us today -- are often tempted to want to be "the greatest." We want bigger, more successful churches, to be more "spiritual," sometimes to have influence and political power. Jesus makes it clear here that the Kingdom -- the way God works with us in the world here and now -- doesn't play by our rules.
I don't think Jesus is glorifying children here. I doubt that he was recommending the view of his day, which saw children as workers and much like property, ways of assuring family wealth, any more than he would recommend the modern American view, in which children are the suns around the lives of their families revolve. Certainly Jesus is making a statement about the intrinsic worth of children (and women, and men, and old people -- of all people). But there's more.
In different eras and societies, children share one characteristic. Children are their essential, true selves, and must learn the roles that their societies socialize them into, whether that of producer or consumer, of subservient or self-actualizing. I hear Jesus saying here that we must become like children -- that we must let go of the rules of this world and the desires for power, wealth and greatness that they engender in order to truly bring about God's kingdom here and now. We must get in touch with our true selves -- our child of God selves -- and let go of the shackles imposed by seeing ourselves as a specific job, or status, or family role. Only then, he says, are we free enough that we can respond to God's invitation to "greatness" -- defined as humility and service -- in the kingdom. This doesn't mean actually leaving all these things, as long as we know where our true self and our true "greatness" lies.