"Holy Thursday" rightly belongs in Blake's Songs of Innocence, for the children are seen as angelic:  they are "flowers" and "lambs," while with "radiance" they are "raising their innocent hands."  In most cases, the Church of England considers Christ's Ascension day to be called "Holy Thursday," (although some insist that "Holy Thursday" can also be the day before Good Friday).  In any event, during the late eighteenth century (when the poem was written), the children from the charity schools of London were marched to a service at St. Paul's Cathedral.  They were, of course, accompanied by the "beadles" who were instructed to enforce appropriate behavior.  The last line is meant to be a direct allusion to the thirteenth chapter of Hebrews that reads, "Do not forget to welcome strangers, for some have welcomed angels unawares" (Hebrews 13:2).  Small poor children seen as angels?  In Blake's Songs of Innocence?  Of course!