Two by two, in red, blue and green, London's children file into St Paul's. Blake's speaker watches them go and is overwhelmed. The poem moves from a quiet, orderly parade to a roar of song and then turns on its reader with a warning: "cherish pity, lest you drive an angel from your door." Its central idea is that these children, who seem to depend on the charity of others, hold a spiritual power the adults around them lack. To neglect them would be to turn away something holy.
The form suits a procession. Each line is long, with a rolling, heavy beat that mimics marching feet, and the lines pair off into couplets just as the children walk "two and two." Most of the rhymes are tidy (clean/green, snow/flow, poor/door), but several are looser: "town" and "own," "lambs" and "hands," and "song" and "among" only half-chime. The effect is of a scene almost perfectly in step but not quite, with a little life breaking through the pattern. The last couplet has the firmest rhyme, and it is the one that gives the order.
The imagery changes as the poem moves. First come colours and cleanliness, a scene that has been arranged. Then the children become water: they "flow" into the cathedral "like Thames waters," a natural force pouring into a stone building. Next they are "flowers," then "lambs," images of freshness and gentleness. The word "multitude" appears twice in the second stanza, and "thousands" follows, so we feel the crowd swelling. The sound swells with it. A "hum" turns into "the voice of song," which becomes "harmonious thunderings." Thunder is a surprising word for children, and it hints that their gentleness is also a kind of power. The word "innocent" opens the first line and closes the second stanza, so that everything in between is wrapped in it.
The adults are placed in contrast. The "grey-headed beadles" carry wands "as white as snow," and the cold colour next to the children's warm ones suggests authority and control rather than life. By the end, the "aged man," called "wise guardians of the poor," sit "beneath" the singing children, who are lifted up toward heaven. The order of power is quietly inverted. Yet the poem does not state its feelings flatly. The speaker says the children "seemed" a multitude, a small word that admits we are looking at an appearance. Calling them "lambs" praises their purity, but lambs are also led and herded. The phrase "guardians of the poor" tells us these children need guarding. A reader can take the poem as sincere wonder, or can hear an undertone of unease about a society that parades its poor children and calls the display holy. Blake also wrote a second poem with the same title, and it takes a far more sceptical view, which makes the question hard to ignore. Still, judging by this poem alone, the awe seems real.
The final line is what lifts the poem above simple description. For three stanzas we have been observers, and then the speaker turns and addresses "you." The line carries an old moral idea, that the stranger at the door may be an angel, and it applies it to the poor of a city. After a scene of beauty, pity becomes a duty, and the threat in "lest you drive" is real. If the children are angels, then failing them has a spiritual cost as well as a human one.
That is why the poem lasts. Its language is simple enough for a child to follow, yet it holds delight and discomfort in the same few lines. It makes us see a crowd of poor children as radiant, and it makes that seeing a test of our own conscience.