And the child which He set in the midst I suppose to have been a very young child indeed, free from all these passions. For such a little child is free from pride and the mad desire of glory, and envy, and contentiousness, and all such passions, and having many virtues, simplicity, humility, unworldliness, prides itself upon none of them; which is a twofold severity of goodness; to have these things, and not to be puffed up about them.
Wherefore He brought it in, and set it in the midst; and not at this merely did He conclude His discourse, but carries further this admonition, saying, “And whoso shall receive such a little child in my name, receives me.”
“For know,” says He, “that not only, if you yourselves become like this, shall you receive a great reward; but also if for my sake ye honor others who are such, even for your honor to them do I appoint unto you a kingdom as your recompence.” Or rather, He sets down what is far greater, saying, “he receives me. So exceedingly dear to me is all that is lowly and artless.” For by “a little child,” here, He means the men that are thus simple and lowly, and abject and contemptible in the judgment of the common sort.
Source: Homilies on the Gospel of St. Matthew (New Advent)