The Chimney Sweeper (Innocence) the effect of the sing-songy rhythm and almost always perfect rhymes? Do they jive with the meaning of the poem?

There is a little iambic, some anapestic with stressed and unstressed syllables thrown in for some fun. The rhyme scheme also adds to a flow that gets the “sing song” vibe you are referring to. The rhymes are really all over the place like “dark” and “work” and warm” and “harm”. It is a fun sounding poem even though the themes, child labour is never a good thing, are dark.