In poetic terms, Blake loved children for their undistorted view of the world and for their innocence. He once claimed that the best or ideal readers of his works (even the really crazy stuff he wrote later in his life) were children.
Because of the value he placed on children and childhood innocence, Blake often writes about children and even features child-speakers, especially in the Songs of Innocence and Experience. "The Chimney Sweeper" is no exception.
Yeah sure the poem's set-up features some boring adult, but the rest of the poem is narrated by the child. Like many of the child-narrators of Blake's poem, he is strangely aware of things like oppression and injustice, but also manages to still be a kid.