The narrator of the poem is advocating for the children but they are addressing two different points of view, that of the children working in the mines and factories, and that of their adult oppressors/ society.
Here the adults are criticizing the children for wanting to die. They seem very dismissive and lack empathy and awareness of the childrens suffering. Theyre telling the children to go play, laugh, and pick flowers in the meadow and in response the children are basically saying that those pleasures are inaccessible to them because unlike their oppressors they dont live a life of privilege, and instead, Their needs go unnoticed in the dark of the coal mines.