It's not tough work imagining a setting for this poem. Our chimney-sweeping speaker is a young British boy in the late 18th century, and he has a lot of chimney-sweeping buddies. They're dirty, they live in squalor, and they have no way out. That's the bottom line.
But when you get into the nitty-gritty of the poem, things start to get a little more interesting. We don't know much about their location (except that they sleep in soot), but we do know what one of them dreams of—a "green plain" (15), "a river" and the "sun" (16). Sounds nice, right?
Right. And that's precisely the point. The contrast between the lovely natural imagery of Tom's dream and black chimneys in which they spend most of their waking life is enough to give anyone the blues. As with many other aspects of the poem, the dream setting highlights the huge gap between the innocence of these young boys and the harsh reality of their experience.