At the close of “The Chimney Sweeper,” young Tom and his companion, the poem's speaker, wake up for another day of dangerous, backbreaking toil.
Despite another day of hard work stretching before him, Tom is in a very good mood. This is because, the night before, he had a powerful, vivid dream in which he and the other chimney sweeps played merrily in the river before going to heaven on clouds. In that dream, the angel told Tom that if he behaves well, then God will take good care of him and ensure that he's happy.
No wonder, then, that Tom's feeling so chipper as he faces another day of working long hours among filth and grime. His positive demeanor appears to have rubbed off on his companion, the poem's speaker, as can be gleaned from the last line:
So, if all do their duty, they need not fear harm.
The correspondence with the promise made by the angel in the dream isn't hard to spot. Just as Tom will be protected and made happy by God if he's a good boy, so he'll come to no harm if he does his duty, which in his case means working hard.
Blake does not actually believe a word of this. These bromides are presented to us as worn-out cliches used to keep the poor and underprivileged in check. Yes, things may be bad on this earth, but if you keep your head down, work hard, and do as you're told, then there'll be a reward for you in the next life.
There's something sad, tragic almost, about the eagerness with which these two young boys swallow this comforting myth. But then, they're too young and too naive, despite their daily experience of life as chimney sweeps, to know any better. And so they head off to work on that cold morning with a song in their hearts.