Theophylact of Ohrid

Thus, not having rejected the children, but having received them with pleasure, the Lord teaches humility "by deed." He also teaches "by word," saying that "of such is the Kingdom of Heaven," those who have a childlike disposition.

Commentary on Luke ↗

Augustine of Hippo

The other doctrine that your brotherhood claims they preach, that little children can attain the reward of eternal life without the grace of baptism, is very foolish. "Unless they eat of the flesh of the Son of man and drink his blood, they will not have life in them." It seems to me that those who claim this for them without regeneration want to nullify baptism, since they teach that these children have what they believe is not to be bestowed on them in baptism even by themselves. …

Letter 182 ↗

Ambrose of Milan

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? …

Catena Aurea by Aquinas ↗

Bede

But Jesus, calling them, said: Let the children come to me, and do not hinder them. For of such is the kingdom of God. He significantly said "of such," not "of these ones," to show that it is not age that reigns, but manners, and that the reward is promised to those who have similar innocence and simplicity. The Apostle also agrees with this sentiment: Brothers, do not be children in understanding, but be infants in malice. In understanding, however, be perfect.

On the Gospel of Luke ↗