What is the effect of the singsong rhythm and almost always perfect rhymes?

The singsong rhythm and rhyme scheme of William Blake's "The Chimney Sweeper" contrast with the dark subject matter. The rhythm of the poem give readers the impression that they are reading something like a children's story or nursery rhyme. However, Blake is critiquing the pitiful conditions under which children labored in the late 1700s to the early 1800s in a rapidly industrializing London.
The poem opens with an overview of the chimney sweeper's tragic beginnings:

When my mother died I was very young,
And my father sold me while yet my tongue
Could scarcely cry " 'weep! 'weep! 'weep! 'weep!"
So your chimneys I sweep & in soot I sleep. (1-4)

The chimney sweeper has lost his mother, and his father "sold" him (presumably for financial reasons and the fact that there was no longer a mother to tend to him). The sweeper now sweeps chimneys and sleeps "in soot." The conditions are atrocious, and the boy is very young.

Later in the poem, the speaker talks about a fellow sweeper named Tom. He had a dream that reveals the dire outlook for these young boys:


And so he was quiet, & that very night,
As Tom was a-sleeping he had such a sight!
That thousands of sweepers, Dick, Joe, Ned, & Jack,
Were all of them locked up in coffins of black; (9-12)



He sees several young boys who were sleepers "locked up in coffins of black." This image captures both the soot-laden environment in which they work as well as the probability that they will die young as a result of the conditions. In the following stanza, the speaker describes an "Angel" that unlocks the coffin and "set[s] them all free" (14). This line represents a childish hope that the boys will awaken to a better afterlife once they've served their time in an earthly hell. The Angel gives Tom the message that if he is "a good boy," he can win the same prize as those boys. The poem ends with the maxim, "So if all do their duty, they need not fear harm" (24). This encourages the children to work, despite the conditions, and hope for something better after this life.

The rhythm and rhyme scheme of the poem both contradict the content and represent the speaker's childish outlook. While the adult reader can see how pathetic the sweeper's situation is, the child can only put his hope elsewhere, knowing that he has no choice but to continue to sweep and sleep in soot.