It is important to focus on the social realities of Blake's times and how he comments upon them through his work. In "The Chimney Sweeper", Blake speaks for the poor children of his day who were forced to do backbreaking labour. In Blake's London, buildings were heated by coal or wood-burning fireplaces, so every house had at least one chimney that had to be cleaned regularly. Poor children were often used to do this dirty and hazardous work because they could fit into the narrow chimney passages. In fact, some parents were so poverty stricken that they sold their children to "masters" who managed crews of young sweepers (as in the accompanying poem in Songs of Experience as well as this one). The work was dangerous, and the children were badly treated by masters concerned only with profits.
In this poem, a child speaker is featured who tries to cheer himself and his fellow chimney sweep, Tom Dacre, with the thought that the oppression and poverty they will endure will be compensated for by endless joy in heaven. Note how notions of children being "sold" by their parents immediately shock us. The description of "little Tom Dacre" is designed to appeal to our sympathy and the repetition of the word "weep" likewise emphasises the harshness of their lives.
However, what is problematic about this poem is the identity of the "Angel" that the speaker dreams of. Are we convinced by the dream of the speaker and his final assertion that "So if all do their duty they need not fear harm"? Is it right that Tom Dacre should go happily back to work, or has he been deluded by an entirely false sense of "duty"? Has he been misled by his own "innocence" about religion and the scheme of things? We could see this poem as an exercise in repression, whereby even the most vulnerable and damaged in society can be convinced by social powers such as religion that they have a part to play in spite of their exploitation. It is part of the mastery of Blake's work that we as readers are left with more questions and answers, and we are unsure of how deep Blake's criticism of his times actually goes.