Jerome

Allow the children, and do not prohibit them from coming to me: for such is the kingdom of heaven. And when he had laid his hands on them, he departed from there. He specifically said 'such', not 'these', to show that it is not age that reigns but character; and that those who have similar innocence and simplicity will be rewarded. The apostle also agrees with this sentiment: Brothers, do not be children in your thinking, but be infants in evil, and mature in your thinking (1 Corinthians 14:20).

Commentary on Matthew ↗

Thomas Aquinas

Consequently he satisfies their devotion: when he had imposed hands upon them. By which he strengthens their virtues. Isaiah 40:29: he gives power to the weary. He departed from thence. Sometimes Christ lays on hands and does not depart from thence; sometimes he lays on hands and departs, because some are so strong that they do not fall back. And he called Peter and Andrew and remained with them (John 1:38ff). Because therefore these were still imperfect, and not capable of following, therefore he departed from thence.

Commentary on Matthew ↗

Remigius of Rheims

Also laying His hands upon them, He blessed them, to signify that the lowly in spirit are worthy His grace and blessing.

Catena Aurea by Aquinas ↗