How do you know that the children in the poem do not like the work they do?

There are several reasons that we know that the children do not enjoy their work in "The Chimney Sweeper: When my mother died I was very young" by William Blake. 

First, the original audience of the poem and most educated readers would assume that working as in indentured servant, living in dire poverty, and spending one's life crawling up and cleaning narrow filthy chimneys is not enjoyable.

Next, in the first stanza the narrator describes...

There are several reasons that we know that the children do not enjoy their work in "The Chimney Sweeper: When my mother died I was very young" by William Blake. 

First, the original audience of the poem and most educated readers would assume that working as in indentured servant, living in dire poverty, and spending one's life crawling up and cleaning narrow filthy chimneys is not enjoyable.

Next, in the first stanza the narrator describes the death of his mother and his being sold by his father as a child so young he could barely speak. The narrator describes Tom Dacre crying. The vision that keeps Tom happy despite the cold and miserable environment in which he works is one of dying, being placed in a coffin, and then going to heaven. What this tells us is that the children are so miserable in their work that they think of dying as preferable.