"What does 'become like a child again' mean? Is the point of comparison a child's humility (cf. Matt. 18.4)? Hardly, as we have no parallel from the world of Jesus for the idea that the child is a model of humility. Or is the point of comparison the purity of children? This idea, too, is not current in early Palestinian Judaism. T. W. Manson's suggestion that Matt. 18.3 could have something to do with the use of the address 'Abba' may put us on the right track. That will, in fact, be the solution. 'Become a child again' means: to learn to say 'Abba' again.
This brings us to the heart of the meaning of repentance. Repentance means learning to say 'Abba' again, putting one's whole trust in the heavenly Father, returning to the Father's house and the Father's arms. Luke 15.11-32 provides evidence that this understanding might not be completely wrong. The repentance of the lost son consists in his finding his way home to his father. In the last resort, repentance is simply trusting in the grace of God."
Joachim Jeremias, New Testament Theology, pps. 155-156