Not to state the obvious or anything, but the poem's called "The Chimney Sweeper" because its about a chimney sweeper. A young, abandoned, crying one, to be exact.
A brief brush-up on the history of chimney sweeping tells us that this wasn't exactly a plumb job. In fact, it was tantamount to slavery in ye olden days of jolly old England, when kids were sold off to handlers, who made them sweep folks' chimneys in exchange for food and shelter.
So while when we read "The Chimney Sweeper," our first thoughts often go to Dick Van Dyke, in Blake's day, this short, to-the-point title would have had a very different connotation. No skipping, no singing. No fun.