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

Blake's "The Chimney Sweeper" shows in various ways that the children do not like the work they do. First, the little boy telling the story mixes up the word "sweep," which the chimney sweepers are supposed to shout in the street to get jobs, with the word "weep," suggesting (although this also is meant to show the little ones lisp) he associates the job with tears and crying.
Then, Tom Dacre does cry when his head is shaved, and he has to be comforted by the narrator. Sadness seems to permeate this job.
Third, the narrator dreams of being released from his "coffin," which is an unhappy image for a bed or a chimney, and being allowed to play outside in the sun and green grass, splashing and getting clean in a river. That this ordinary outdoor activity is dream that is out of reach of the chimney sweeps shows how harsh their young lives are.
At the end of the poem, innocent Tom is happy despite the cold and dark because of his dream: it is not anything to do with his job or real existence that brings him joy, but a fantasy world that is entirely different from his reality.

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.