Hello,
You may have long since solved your mystery, but in case you haven't, I did find them in the 1871 Census:
Ann Walker 46 Manchester, Lancashire, England Wife Oldham Lancashire
John Walker 18 Staleybridge, Cheshire, England Son Oldham Lancashire
Joseph Walker 44 Peak Forest, Derbyshire, England Head Oldham Lancashire
Mary Ann Walker 21 Manchester, Lancashire, England Daughter Oldham Lancashire
Wm Thos Walker 13 Accrington, Lancashire, England Son Oldham Lancashire
Living at 233 Lee Street, Oldham.