These are the chiastic structures in Mat 18 that I have found so far:
1a) Mat 18:1, Who is the greatest in the kingdom of heaven;
1b) Mat 18:2, He called a little child and set him in their midst;
central axis) Mat 18:3, Assuredly, I say to you, unless you are converted and become as little children, you will by no means enter the kingdom of heaven;
2b) Mat 18:4a, Whoever humbles himself as this little child;
2a) Mat 18:4b, Is the greatest in the kingdom of heaven.
1a) Mat 18:5, Whoever receives one little child in My name receives Me;
1b) Mat 18:6, Whoever causes one of these little ones who believe in Me to sin;
1c) Mat 18:7a, Woe to the world because of offenses!;
central axis) Mat 18:7b, For offenses must come;
2c) Mat 18:7c, Woe to that man by whom the offense comes;
2b) Mat 18:8-9, Cast away the causes of sin;
2a) Mat 18:10, Take heed that you do not despise one of these little ones.
1a) Mat 18:11, For the Son of Man has come to save that which was lost;
1b) Mat 18:12a, If a man has 100 sheep and one goes astray, he leaves the 99;
central axis) Mat 18:12b, He goes to the mountains to seek the one that is straying;
2b) Mat 18:13, If he should find it, he rejoices more over the one than over the 99;
2a) Mat 18:14, It is not the will of your Father in heaven that one of these little ones should perish.
1a) Mat 18:15, If a brother sins against you;
1b) Mat 18:16-17, By the mouth of two or three witnesses every word will be established;
central axis) Mat 18:18, “Assuredly, I say to you, whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven;”
2b) Mat 18:19-20, Two or three gathered together in Jesus’ name in agreement;
2a) Mat 18:21-35, Parable of the unforgiving servant.
There is so much wisdom to be gained by meditating on the central axis’ and matching pairs! If you would like, leave your insights in the comments, please.