This is from the LDS transcription of the 1881 census:
South Street Marnoch, Banff
John FEARNSIDE Head 48 Teacher of Music b Fintray, Aberdeen
Eliza FEARNSIDE Wife 43 Wife Of Teacher Of Music b Longside, Aberdeen
Eliza Ann FEARNSIDE Daur 21 Mothers Assistant b Keithhall, Aberdeen
Caroline Jane FEARNSIDE Daur 17 Pupil Teacher b Dublin, Ireland
Robert FEARNSIDE Son 14 scholar b Old Deer, Aberdeen
Stephen FEARNSIDE son scholar 12 b Old Deer, Aberdeen
May FEARNSIDE daur 10 scholar b Old Deer, Aberdeen
Williamina FEARNSIDE dau 8 scholar b Old Deer, Aberdeen
Anna FEARNSIDE dau 7 scholar b Old Deer, Aberdeen
Helen FEARNSIDE daur 5 b Old Deer, Aberdeen, Scotland
Ida FEARNSIDE granddaughter 3 b Aberchirder, Banff
Martha FEARNSIDE daur 1 b Aberchirder, Banff, Scotland
Using the free search facility on FamilySearch, it would seem that mother Eliza's surname was Smith
Eliza Ann Smith Fearnside daughter of John & Eliza (nee Smith) Fearnside was b 3 Aug 1859 & baptised 28 Aug 1859 at St Mary's Episcopal church in Inverurie.
Ida's birth has an additional name Cheyne in the SP index (no credits spent). She appears as an adopted daughter in the household of Alexander & Eliza Monro in 1891. Checking the marriage index on a free search at SP Eliza was Eliza Fearnside so Ida is her daughter. Martha still with John & Eliza in 1891 listed as daughter.
Again from a free search of SP. John Fearnside m Eliza Smith 8 August 1854 at Old Deer in Aberdeenshire.
Nell