Have you found Martha and her children in 1871? Is it possible that she died prior to 1871?
Is this Martha’s daughter?
1871 Linkenhorne, Cornwall, England Census
Elizabeth Aunger, age 12, born St. Ive, Cornwall
On census with her grandparents and aunt
Martha’s son?
1871 Liskeard, Cornwall, Census
John Aunger, age 15, born Liskeard, Cornwall, Farmservant
On census with Edward Grigg and family
Added - Martha (age 42), Elizabeth (age 8 ) and George (age 12) were in Liskeard, Cornwall in 1871 (under Aunges). Possibly your Martha and children?