When we first meet "The Chimney Sweeper," he is alone in the snow. It sure looks like he's abandoned. At the end of the first stanza, and again in the third stanza, the sweeper talks about how his parents have left him to go to church and pray. Plus, we can't help but wonder if this little guy isn't a chimney sweeper precisely because his parents decided to make him one. And isn't that a kind of abandonment, too? They may be around, sure, but they've left their kid to be exploited by an unfair industry.
Abandonment need not be literal; "The Chimney Sweeper" argues that forcing kids to work, ignoring them to go to church, and the like are just as bad as actually ditching them altogether.
In this poem, the abandonment isn't a conscious decision. The parents abandon their kid because their priorities are out of whack. They think it's more important to pray than to feed your kid.