The first version of "The Chimney Sweeper" appeared in Blake's collection Songs of Innocence (1789). Need we say more? Yes, as it turns out. That's because this particular song is all about the absence of innocence. The kids in this poem have no childhood whatsoever. They get up before dawn and clean chimneys. In that sense, their innocence has been stolen from them. They're forced to live a "black" life, covered in soot and facing a premature death. They frolic and play only in dreams.
The boys may live black lives, but they're still innocent. The fact that Tom believes that if he does his duty, no harm will come to him shows that he's still hanging on to his childhood despite his awful life.
"The Chimney Sweeper" argues that money is to blame for destroying these kids' innocence. After all, the speaker's childhood is taken away after he is "sold."