Stanza 1 Summary

Get out the microscope, because we’re going through this poem line-by-line.

A little black thing among the snow,
Crying "'weep! 'weep!" in notes of woe!

Okay, okay, so this isn't exactly a sentence. Still, we can work with it. These lines seem to be an observation.
The speaker sees a little black thing in the snow. And that little black thing is sadly crying "'weep! 'weep!"
That's all well and good, but what in the world is this little black thing? A person? An animal? We're gonna go ahead and guess that it's the subject (and title) of the poem—a chimney sweeper.
That also means that this little black thing is a young boy, because in Blake's day, that's who swept chimneys.
The fact that this little guy is referred to as a thing is telling. The speaker doesn't even see him as a person. Harsh.
The chimney sweeper probably looks black because he's covered in soot.
There are more than a few things we can notice about these lines, poetically speaking.
First, there's the stark contrast between the soot-covered boy and the pure white snow he's sitting in. That's quite an image.
Then there's the rhyme—snow and woe. That's a perfect rhyme, nice and neat, and since these two lines go together, we call this a rhyming couplet.
Notice anything else? Of course you do: the meter. It goes a little something like this: daDUM dadaDUM daDUM daDUM. Be sure to head on over to "Form and Meter" for more on this lilting rhythm.

"Where are thy father and mother? Say!"--
"They are both gone up to the church to pray.

Instead of leaving the poor kid alone, the speaker asks him a question. He seems a bit concerned for the little guy, because he wants to know where his parents are.
But he's also more than a little abrupt. He practically demands the kid tell him what's up ("Say!")
The chimney sweeper says that his parents have gone to church.
And there's that perfect rhyme again. Say and way. Look out for this pattern of rhymes to continue—or be broken.