Theophylact of Ohrid — as excerpted in the Catena Aurea on Mark 10:13-16

“Wherefore He says not, for of these, but of such is the kingdom of God, that is, of persons who have both in their intention and their work the harmlessness and simplicity which children have by nature. For a child does not hate, does nothing of evil intent, nor though beaten does he quit his mother; and though she clothe him in vile garments, prefers them to kingly apparel; in like manner he, who lives according to the good ways of his mother the Church, honours nothing before her, nay, not pleasure, which is the queen of many; wherefore also the Lord subjoins, Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child, he shall not enter therein.”