The immolation of innocent children in the capital city of Jerusalem was an important reason for Jeremiah's prophetic denunciation of his contemporaries in a message delivered in the Hinnom Valley, the infamous location of the Topheth. Standing there, the prophet proclaimed that Yahweh had said that "they constructed the high places of Baal to burn their sons in the fire as burnt sacrifices to Baal, which I neither commanded, articulated, nor did it rise up in my mind" (Jeremiah 19:5).
Some of the arguments were that Yahweh never endorsed the practice at the Topheth (Jeremiah) and that he temporarily commanded the Israelites to practice firstborn child sacrifice (Ezekiel) but that he did not endorse it completely.
Blake's phrasing relates to the Hill of Giants as clarified by Joshua (15:8), for "the valley of the son of Hinnom [which means hell], the hill of Topheth
[which means furnace] ...