Blake's "The Chimney Sweeper" is a powerful indictment of a society that puts making money above its virtual enslavement and abuse of its children. Though the boys are not to blame--they are children, after all--their naivete leads them to accept what society tells them and allows them to be pacified. If ever there was a good example of Marx's idea that religion is the opiate of the people, the chimney sweeps in this poem are that example.
Tom's hair is like that of a lamb, in that he is innocent and, like lambs are traditionally thought of, a victim (a lamb being led to slaughter). He will spend his days in confining, black, dark chimneys (the coffins of the dream) and his only escapes are dreams and death. And the soot in his lungs will almost certainly lead to an early death. The sweeps are sold into servitude and made to think that it is okay, since their imprisoned-like state will guarantee them a place in heaven. If they behave in this life (go along with the abuse without arguing) they will be rewarded in the next life: very convenient for those in power.
Blake condemns society in this poem. The children are mistaken in their naive, childish acceptance of their fate, though again they are not to blame: business people, political leaders, and the church are to blame.
The version of "The Chimney Sweeper" in Blake's Songs of Innocence describes Tom Dacre who is compared to a sheep early in the poem. In the poem, Tom is a symbol of innocence and traditionally, lambs are also used to represent innocence. As the poem continues, Tom dreams of his friends who are trapped and freed by an angel. When he awakes, he goes off to his job in a jolly mood. Throughout the poem, Tom's childhood innocence is threatened yet he manages to maintain it even though he is already part of the labor force. So, Tom's being compared to a lamb creates a foundation for one the major themes of the poem--the quest to maintain a childhood innocence.