The poem describes the annual Holy Thursday (Ascension Day) service in St Paul's Cathedral for the poor children of the London charity schools. The children enter the cathedral in strict order 'walking two and two' behind the beadles (wardens). The children sit and sing, and their voices rise up to heaven far above their aged guardians. The poem ends with a moral: have pity on those less fortunate than yourself, as they include angelic boys and girls like those described here.

The poem is based on the contrast between the 'innocent faces' of the children and the authority of the 'grey headed beadles' and the other 'aged men' who act as their guardians. Although the children are made to enter the cathedral in regimented order, their angelic innocence overcomes all the constraints put upon them by authority - they even make the 'red and blue and green' of their school uniforms look like 'flowers of London town'. As the boys and girls raise their hands and their voices to heaven, the narrator imagines them rising up to heaven too, just as Christ himself did on Ascension Day. In the poet's vision they leave their 'wise Guardians' beneath them and become angels - which is why the last line tells us to 'cherish pity' and remember our duty to the poor. Although the triple repetition of 'multitude(s)' notes how many thousands of children live in poverty in London, the emphasis in this poem is on the 'radiance' which they bring to the church - they are 'multitudes of lambs'. We must wait for the contrary 'Holy Thursday' poem in Songs of Experience for Blake's social critique: 'And so many children poor? It is a land of poverty'.