For the last line, Blake originally wrote, "Who wrap themselves up in our misery." The mended line highlights a central feature of the poem. Had he written "wrap themselves up in our misery," it would have contrasted powerfully with the soot-impregnated clothes cum winding sheet of the child. But the line as it stands clutches at us beyond its intratextual ironies; it makes a point, to be pointedly old-fashioned about it. The social constitution of heaven--a "made up" paradise--is brought to the fore. Nor is this the pie-in-the-sky religiosity of Tom's dream in "The Chimney Sweeper" of Innocence. Rather it's a heaven forcibly constructed by the power of "God & his Priest & King," and by the parents who've bartered their child into a miserable and probably deadly occupation.
And it is the sweeper's misery--precisely his misery--that gives form to that made-up heaven. The child's recourse to the lost world of innocence makes it possible for society to ignore or discount its part in the blighted drama of the sweeps' lives ("And because I am happy, & dance & sing / They think they have done me no injury"). But at the core of 18th-century English society, with its soon-burgeoning imperial and capital-intensive interprise, its increasingly impermeable class structure and stolid faith in the Thirty-nine Articles, lay the countervailing world of child labor, wage slavery, and religious dissent. The potentiality of that world never materialized in blunt revolution; it remained for men and women like Blake and Wollstonecraft to shine a light on it.