William Blake's "London" presents the eponymous city of London as a dark, hopeless, miserable, and poverty-stricken place. The speaker references children, or "Infants," twice in the poem. He does so to emphasize the degree of the misery and hopelessness, which impacts even these children. He also uses children in the poem to symbolize the innocence and purity that can no longer exist in the city of London.
In stanza two, the speaker says that he hears "The mind-forg'd manacles" in every voice in the city, and even in "every Infants cry of of fear." The "mind-forg'd manacles" are a metaphorical reference to the restrictions that inhibit and, metaphorically, imprison the lives of the impoverished citizens of London. Their freedom is limited because of their poverty. The "Infants" symbolize innocence and purity, and so the fact that even infants, or children, are crying with fear, implies that the poverty and misery of London has impacted even the most vulnerable, and at the same time destroyed any innocence or purity.
In the fourth and final stanza, the speaker references "Infants" once more. He describes the "midnight streets" which echo with "the youthful Harlots curse," and he says that this "curse" in turn "Blasts the new-born Infants tear." A harlot is another name for a prostitute. The fact that the harlot is "youthful" is tragic, but the implication that there is a child born of this prostitute and that the first sound that this baby hears is the curse of its mother, is especially tragic. The word "Blasts" also connotes violence, and the word "tear" of course connotes sadness. The overall implication is that innocence and purity, as symbolized by the infants, cannot survive in the city of London.