Okay, okay, so "The Chimney Sweeper" isn't very happy. but the chimney sweeper mentions happiness several times. At first, the sweeper says he used to be happy, before he was mistreated by his parents and forced to clean chimneys. But then, in the final stanza, he says he is still happy in some sense. This raises the question: what does it mean to be happy for a kid who's forced into the chimney-sweeping life?
The kid is happy (and yes, he's really happy), because kids are resilient, and that's what this poem is really about.
This kid isn't happy at all. He's just faking it to save his parents the trouble of doing anything about it.