If you have read Charles Dickens's classic novel, Oliver Twist, you will remember that Oliver was almost apprenticed to a Mr. Gamfield who worked boys as chimney sweepers until gentlemen object,

"It's a nasty trade," said Mr. Limbkins, when Gamfield had again stated his wish.
"Young boys have been smothered in chimneys before now," said another gentleman.

Like Dickens's subtle invective against the occupation of boys as chimney sweepers, William Blake, too, exposes the horrible cruelty of this occupation.  It is Blake's intention as well to bring to the public's attention the need to do away with such a practice that exploits small boys.  In 1788 there was a Chimney Sweepers' Act passed; however, it only marginally improved conditions for the boys.

So, with all this in mind, your thesis could state that William Blake writes a poem of social protest for the innocent in his "The Chimney Sweeper."  The three opinions of this thesis could reflect what is contained as main points in the poem. Another idea is to have the thesis state how Blake demonstrates in "The Chimney Sweeper" demonstrates how the society of his day puts commercial values ahead of moral ones.  Then, the three points of the thesis can be in which ways this demonstration is accomplished. 

e.g. In his poem, "The Chimney Sweeper," William Blake demonstrates how the English society of his day places commercial values ahead or moral ones by ____.______.and _____.  [find verses that answer HOW]