A little black thing in the snow
Crying "weep! weep!" in notes of woe!
"Where are thy father and mother? Say!"
"They are both gone up to the church to pray
"Because I was happy upon the heath
And smiled among the winter's snow
They clothed me in the clothes of death
And taught me to sing the notes of woe
"And because I am happy and dance and sing
They think they have done me no injury
And are gone to praise God and his priest and king
Who make up a heaven of our misery."
In the previous line,  the child in the poem is thought to put on a successful façade for others to believe he is truly happy with their work. Due to the fact the child “dance[s] and sing[s]”, people believe nothing is wrong. They ‘dance and sing’ — a metaphor perhaps for play — in an effort to retrieve some joy in life 
 “No injury” refers to the child calling out to people who think everything is well. He mentions that people think they have done him no harm, despite his misery and despair.
The Chimney Sweeper Songs of Experience