I have had difficulty finding birth registrations for all of those Day children from the census (Post 16). For the few I found the Mothers maiden name comes up as PERKINS.
Marriage 25 Dec 1876 St Mary, Rotherhithe, Southwark. (Anglican marriage)
James Day (22 Woodcutter) and Margaret Perkins (20 spinster) Both living close to each other in Osprey Street Rotherhithe.
His father Henry Day, woodcutter
Her father George Perkins, Labourer
(Note the woodcutter employment is of interest as Margaret is in that employment in a later census, and also some of the children)
I have to go out now, so will let others continue the search.