What blocks us from the little child ethic Jesus delighted in? Said another way, What is the opposite of entering the kingdom like a little child? If little children prompted Jesus to say, such is the kingdom, what, then, is not the kingdom.
Seeming to target this sort of thing, Paul writes, The kingdom of God is not a matter of eating and drinking (in context, in a way destructive of a brother) but of righteousness and peace and joy in the Holy Spirit. (Ro. 14:17 ESV) Similarly, the Apostle John suggests, when closing his epistle, it is anything that competes with delighting in God: Little children, keep yourselves from idols. (1 John 5:21 ESV) In line with these, and perhaps the perfect negative example, the exact opposite, of the little children model, is the rich young ruler. In the midst of his teaching on little children, Jesus interacts with the rich young ruler. Here is perhaps the ultimate one who won't, and really can't, be like a little child.
The Young Ruler
The young ruler is so intent on successful performance that he simply can't receive the blessings Jesus had so freely given the little ones brought to Him. It is his performance focus, so ingrained in the young ruler, that blocks the contentment needed for the blessing of the Spirit.
It's helpful (at least for me) to think of the NT teaching on the kingdom as having two aspects: the kingdom as embedded in the Spirit, and the kingdom that will be encountered at His coming. I.e., the rapture, and right now, when a Christian is in sync with the Spirit and real kingdom work is performed. Usually the texts seems to tilt to one or the other rather than encompassing both aspects.
The young ruler's problem is being locked outside the flow of the Spirit, humility and contentment, and therefore kingdom work. Because in this case Jesus speaks of "never" entering the kingdom, it seems clear the young ruler needs first regeneration, as well as the Spirit-led productivity that lays up secure treasure. Throughout the conversation the Lord is intent on answering the rich young ruler's original question, which, translated, was the desire for inheritance, wealth in the afterlife.
The Escape Route
Jesus doesn't berate the rich young ruler for this desire, but sees through his performance obsession and pities him. In order to rescue him, Jesus gives the young man three hard truths.
(1) Jesus presses him on what is good, which is God alone. Like Paul telling the Philippians to focus on what is just, pure and commendable, Jesus tells the rich young ruler to get his head into God and into His immersion of blessings, to rejoice in the goodness of God. He is revealing that even the Son of God wasn't self-focused but delighted in the goodness of His Father.
(2) He enumerates the commandments of the Law to prod him to be poor in Spirit, to mourn. The Lord is advising him to let the Law create in him a godly grief that will lead to holiness. Born again Christians are freed up by this realism about themselves. Yes, it is true per Galatians that when we are in the Spirit we are free from the law, but it is painfully true that when we are in the flesh we are convicted as lawbreakers. If this is true for us as Christians, certainly those who are not born again are also convicted as lawbreakers. For all of humanity, conviction of sin is the vivid grief that is the first glimpse of the vista that can become growth in grace.
(3) When the rich young ruler reveals his hardened defenses by claiming to have kept the Law, Jesus (gallantly, I think) leads him to an emergency escape route: Dive into the worship seen in little children by becoming actually and totally dependent. Come, follow Me. Become dependent for real. Like the psalmist, become forcefully, single-mindedly, desperately in a state of dependence (real and not theoretical) on God.
But the young man can't and won't because he is, instead, locked outside with his idols. "To deny oneself is to be aware only of Christ and no more of self, to see only Him who goes before and no more the road that is too hard for us," Dietrich Bonhoeffer describes a freedom that the young ruler knew nothing of, the freedom of hard obedience, that comes, not from prioritizing God, but by becoming one of God's precious priorities. In one fell swoop the follower leaves behind his nets and "many things" per Martha, and, with the silent adoration of Mary, follows.