The speaker's history in Blake's "The Chimney Sweeper" comes in the first stanza:
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.
The repeated weep in the third line represents a child trying to say sweep, but being unable to, and of course is also a play on his crying for having to leave home. He was very young, in other words.
This was a common situation in the England of Blake's day. Some sweeps were orphans, but many were sold by families that couldn't afford to raise them.
The job of chimney sweep was very nearly a death sentence: the soot in a person's lungs often killed a sweep sooner or later.
This speaker is naive and goes on to tell how he and other boys accept their fate because they will be rewarded later in heaven, going along with the rationale given to them by adults and the church for allowing such mistreatment.