A child of perhaps seven is the speaker of Blake's subtle poem. For, it is with a poignant naivete that the boy relates the trials that the "climbing boys," as they were called, suffer. And, with the use of this child's voice, Blake craftily points to the horrible conditions under which these boys work when the only solace they can seek is that in dreams or in death.
And by came an angel who had a bright key,
And he opened the coffins and set them all free;
Then down a green plain leaping, laughing, they run,
And wash in a river, and shine in the sun.
In addition, Blake is able to create a pun with the little boy's mispronunciation, something he could not have done if his voice were that of the speaker.
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.
Thus, he plays upon the association of the sweepers' weeping because of their pitiful existence.
The speaker in the poem is the child, who is a chimney sweeper. There are two poems by Blake with the same title. The child is the speaker, though some people will argue that there are two speakers or that one is the child and the other is the child's adult voice.