Ambrose of Milan — as excerpted in the Catena Aurea on Luke 18:15-17

“It may be thought strange by some that the disciples wished to prevent the little children from coming to our Lord, as it is said, when they saw it, they rebuked them. But we must understand in this either a mystery, or the effect of their love to Him. For they did it not from envy or harsh feeling towards the children, but they manifested a holy zeal in their Lord’s service, that he might not be pressed by the crowds. Our own interest must be given up where an injury is threatened to God. But we may understand the mystery to be, that they desired the Jewish people to be first saved, of whom they were according to the flesh. They knew indeed the mystery, that to both nations the call was to be made, (for they entreated for the Canaanitish woman,) but perhaps they were still ignorant of the order. It follows, But Jesus called them unto him, and said, Suffer little children, &c. One age is not preferred to another, else it were hurtful to grow up. But why does He say that children are fitter for the kingdom of heaven? It is because they are ignorant of guile, are incapable of theft, dare not return a blow, are unconscious of lust, have no desire for wealth, honours, or ambition. But to be ignorant of these things is not virtue, we must also despise them. For virtue consists not in our inability to sin, but in our unwillingness. Childhood then is not meant here, but that goodness which rivals the simplicity of childhood.”