Mark has included John’s question in verses 38-41 which interrupts Christ’s teaching about true greatness, but now after answering John, the Lord returns to this subject, so that verse 42 should be seen as following on from verse 37. It is a continuation of his correction to their wrong attitude in disputing who would be the greatest.
The Scripture teaches possibility of childhood faith and parents should teach their children the word of God from the earliest age (2 Timothy 3:15). But the child is weak and although God will not lose a single one of those who have truly believed in him, he is particularly vigilant, if we may say it, to watch over those who are weakest. The same certainly applies to all who are not strong in themselves but who rely on God, so that the literal child may be taken to stand for every child in the faith. It is a beautiful picture of God’s attentiveness to those who most look to him for help.