In the synoptic Gospels that is to say in the Gospels of Mark, Matthew and Luke there is a reference to the children who visit Jesus. There is no reference in the Gospel of John about the encounter of the children with Jesus.
In the Gospel of Luke, Jesus told the disciples when he received the children: "Let the children come to me, and do not hinder them, for the kingdom of God belongs to such as these. Truly, I say to you, whosoever does not receive the Kingdom of God like a child shall not enter in. Luke 18:15-17
The Kingdom of God is the transformation of man towards his nature, that is, the conformity to God. According to the Bible, man is good by nature. Every being tends to its own good or perfection explains Thomas Aquinas. Example: the house is good when it is finished, the fruit is good when it is mature.
In this transformation of man there is a second birth, that is why "we are children again" or in other words "we are like children twice".
In the Gospel of John, Jesus tells to Nicodemus:

Flesh gives birth to flesh
but the spirit gives birth to the spirit
Yo should not be surprised at my saying:
You must be born again.
John 3:6-7

There are two births, one is the birth of the flesh, the physical birth; the other is the spiritual birth, the second birth.
This second birth is to receive the Holy Spirit, the essence of God, in philosophy this birth is called Gnosis. Through this birth we are wise as God is wise. This birth is repentance of heart asking God for wisdom, through it we receive the 7 spiritual gifts of the Holy Spirit and we are kids again, we became "spiritual children".
Is like this that we receive the kingdom of God, being persons renewed by the spirit, and this renewal has a way, the gift of serenity or fear of the Lord, also known as constancy, firmness, devotion.
The parable of the sower teaches us to be children again, receiving the word of God (the knowledge of the correct means to live), retaining it (having allways in mind) and practicing it with constancy, is so that we achieve the interior lighting, the likeness to God.