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
O what a multitude they seemed these flowers of London town
Seated in companies they sit with radiance all their own
The hum of multitudes was there but multitudes of lambs
The simile emphasises the flow of children — several thousands — the numbers of orphans in London was huge. 
 The Thames reference is ironic. The verb ‘flow’ suggests the sort of symbolic ‘freedom’ that belongs to the river, but not to the poorer class in a society that was minutely controlled by the power of the ruling elite and the church. 
 We might question the comparison with Thames waters. Is this a comparison with the pure natural world? Or should we bear in mind that the Thames was polluted; an open sewer? It is for the reader to speculate.
Holy Thursday Songs of Innocence