The Chimney Sweeper from the Songs of Innocence is very different from the one from the Songs of Experience. The first link summarizes the Innocence poem. It is about a little boy who has been sold at a very young age into this sort of slavery of cleaning chimneys with other young boys. In this poem he has an optimistic perspective and believes that if he works hard and does his duties, then he will go to heaven. In other words, this is a naive and innocent little child who does not see the world as harsh and unforgiving. If he keeps his faith, he'll go to heaven someday.
In the second Chimney Sweeper (from the Songs of Experience) the boy's attitude is very different. He is very much aware of what his parents have done to him. They have sold him into this "slavery" and while he works this horribly dangerous job, they are off at church. This same church is the institution that supports what his parents and other parents are doing to all children in this era. The was before any children had rights. There were no laws against children doing labor like we have today. This boy saw the world/society for how cruel it really was, and he is calling everyone out for it.