Jesus uses this parable of the children (and the others) to convey how he wants the apostles to behave and teach others to behave and believe.
It includes the parables of The Lost Sheep and The Unforgiving Servant which also refer to the Kingdom of Heaven. The general theme of the discourse is the anticipation of a future community of followers, and the role of his apostles in leading it
In Chapter 19 we observe:
(KJV) Matthew 18:3 And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.
4 Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven.
5 And whoso shall receive one such little child in my name receiveth me.
6 But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and [that] he were drowned in the depth of the sea.
...where he has a brief dicussion with the Pharisees again, performs some miracles, teaches his disciples, and intimates further about the Kingdom to come:
(KJV) Matthew 19:1 And it came to pass, [that] when Jesus had finished these sayings, he departed from Galilee, and came into the coasts of Judaea beyond Jordan;
The good sayings continue into Chapter 20, where he eventually departs Jericho and restores the sight of two blind men. I won’t belabor it or Chapters 21, 22 & 23 except to say that Jesus continues parabolicizing. That these passages are not considered “sermons” as the 5 Discourses are, seems kinda nitpicky. If you’re a believer, there is good reason to pay attention.
(KJV) Matthew 19:28 And Jesus said unto them, Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.
29 And every one that hath forsaken houses, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for my name's sake, shall receive an hundredfold, and shall inherit everlasting life.
30 But many [that are] first shall be last; and the last [shall be] first.
When I continue, the sermon on the Mount of Olives.