Twas on a Holy Thursday, their innocent faces clean
The children walking two & two in red & blue & green
Grey headed beadles walked before with wands as white as snow
Till into the high dome of Pauls they like Thames waters flow
Blake begins in story-telling mode, setting the tone of spirituality and purity.   Ascension Day , when Christ rose to heaven, is appropriate for this idealised event involving the children. There is irony in that they would have been far from ‘innocent’, having experienced parental abandonment and ill-treatment. Given the appalling conditions in which they lived, the children may not have been clean either.
Holy Thursday Songs of Innocence