If you've taken a look at Deborah Noel's bibliography (linked from the Title Page), you've probably noticed that there's not much agreement about what the poem is "about." Like so much of Blake's poetry, "The Chimney Sweeper's" ambiguities dissolve and shift under the weight of interpretation.
Right down to the phonemic level, we're faced with a provisional choice between the elided street cry of a child too young to announce his trade, and the uncoerced "weep" of a mere infant. They might both be true, in the sense that a pun is "true."
Certainly, Blake is fond of puns: they turn up again and again in his poetry. But puns of the conventional sort derive wit from the yoking together of incommensurable terms: in a sense, they rob "truth" or certitude from both sides, and repay the debts with an irreducible possibility. Blake's punning is of an altogether different sort; it's rather more like a parable, in which meaning sidles outward from a linguistic center. This may be why the best of Blake's critics have been "strong" readers, readers who feel some sense of confidence, even when, as Geoffrey Hartman puts it, "with Blake, we're never quite sure" (Hilton, p. 245).
There's another sense, though, in which both the "weep" of the text and its implied "[s]weep" are at once true. They track a linguistic gradient of the commodification of the self. For a Londoner of Blake's time, particularly for a tradesperson who, as an engraver, was working at the very cusp of mechanical reproduction, the encroachments of expanding civic capitalism would have been keenly felt. In the poem's patent terms, a child whose mother has died is suddenly worth something. The scene of primal attachment, of child to mother, is displaced by an eruption of the underlying system of property rights and barter. The repeated "weep weep weep weep," with its introduction of metrical difficulty into the poem, is no mispronunciation, but rather, a clear articulation of the commodity fetish to which the child--no longer child, but chimney sweeper--must succumb.
And the reader knows. "So your chimneys I sweep & in soot I sleep." The reader, indeed, is caught in the same web of relationships broken by deaths, and remolded into fungible shapes by the plasticising Invisible Hand of capital.