Like the Sermon on the Mount, vs. 10 show us that even the attitude of looking down on a little one is sin. Not trying to win back your brother who has sinned against you. Not using God given authority appropriately. Not uniting with the mind of Christ in our common prayers so as to establish the maximum welfare for God's people. Not forgiving your brother from the heart.
Sin is serious.
For I tell you that their angel in heaven always see the face of my Father in heaven. Not clear what it means. Obviously something shared in Matthew's worldview by his original readers. Weight is quite clear. They've got God himself on their side, and God himself is their life and glory, so be careful whom you take on.