The child tells how his father sold him to a master sweep when he was so young that he could not even pronounce the words 'sweep, sweep' (the traditional street cry which chimney sweeps called out to advertise their presence). The boy comforts Tom Dacre, another sweep whose blond hair has just been shaved off. Tom goes to sleep and dreams that an angel sets free all the sweeps so they can run, play and swim freely in the innocence of youth. The angel tells Tom that if he is a 'good boy' God will love him and he will never 'want joy' (lack happiness). Tom awakes, warm and cheerful, and the poem ends with the moral: 'So if all do their duty, they need not fear harm'.

In Blake's time, poor parents often sold their children as 'climbing boys' to a master sweep at around the age of five. The boys were forced up narrow, winding chimneys to clean them of soot. Some suffocated inside the chimneys they were trying to clean; others grew up stunted and deformed, dying at a young age from cancer or lung diseases. Tom Dacre's dream shows just how horrible this life was for the boys by contrasting it with what they should have been doing at this tender stage in their lives: 'leaping' and 'laughing' in the sunshine. The moral at the end of the poem is the statement of the young sweep who narrates the poem. Obviously it is nonsense: the climbing boys all 'do their duty' but still come to great harm. Yet the sweep is just innocently repeating the moral code which he has been taught by society. The poem thus holds a mirror up to its readers: it is you who deceive children with this false morality, just as it is 'your chimneys' (verse 1, line 4) that are responsible for having boy sweeps in the first place.

Compare this poem with its 'contrary', 'The Chimney Sweeper' in Songs of Experience.