That he passed by some boys, so he greeted them with Salām and said that the Prophet (may Allah's peace and blessings be upon him) used to do so

This Hadīth encourages the Muslim to be humble and to spread the greetings of Salām to everyone. It also shows the Prophet's humbleness as he used to greet young children while passing by them, and his Companions (may Allah be pleased with him) followed him in doing this. Anas (may Allah be pleased with him) used to pass by the children and greet them.

He used to pass by them in the market while they were playing and he would greet them with Salām, and he said that the Prophet (may Allah's peace and blessings be upon him) used to do the same thing, i.e. greet the children while passing by them. This is a reflection of the humbleness and exemplary manners of the Prophet, and a manifestation of his approach in teaching and guiding the children.

If grown ups greet children, they become accustomed to this and it eventually becomes their instinct habit.