Luke 18:17

“Truly I say to you, Whoever shall not receive the Kingly Rule of God in the same way as a little child, he shall in no wise enter into it.”

And then He adds a solemn saying, as evidenced by its opening words, the solemn ‘truly I say to you' which occur only seven times in Luke. And the point of His saying is that anyone who receives the Kingly Rule of God must do so in the ready and willing way in which a little child does. For there is no other way to receive it. These children would have no hang ups about obeying God, they would see it as the right thing to do (even though they might sometimes forget it on the spur of the moment). It is only adults (over elevens) whose hearts, like that of the rich young ruler in the next story and the Pharisee in the previous story, become hardened against obedience to Him.

Arguments about whether receiving the Kingly Rule of God refers to the present Kingly Rule, present in Jesus, or the future heavenly Kingly Rule are unnecessary. It is the whole concept of the responsiveness of hearts towards God that is in mind, and that includes both this world and the next.