Therefore He said, “of such is the kingdom of Heaven,” that by choice we should practise these things, which young children have by nature. For since the Pharisees from nothing else so much as out of craft and pride did what they did, therefore on every hand He charges the disciples to be single hearted, both darkly hinting at those men, and instructing these. For nothing so much lifts up unto haughtiness, as power and precedence. Forasmuch then as the disciples were to enjoy great honors throughout the whole world, He preoccupies their mind, not suffering them to feel anything after the manner of men, neither to demand honors from the multitude, nor to have men clear the way before them.
For though these seem to be little things, yet are they a cause of great evils. The Pharisees at least being thus trained were carried on into the very summit of evil, seeking after the salutations, the first seats, the middle places, for from these they were cast upon the shoal of their mad desire of glory, then from thence upon impiety. So therefore those men went away having drawn upon themselves a curse by their tempting, but the little children a blessing, as being freed from all these.
Let us then also be like the little children, and “in malice be we babes.” For it cannot be, it cannot be for one otherwise to see Heaven, but the crafty and wicked must needs surely be cast into hell.
Source: Homilies on the Gospel of St. Matthew (New Advent)