Your census dates seem to be incorrect. John and Robert Allen, aged 9 and 10 respectively, appear in the 1871 (not 1861) census living with the Russells. They are not noted as servants -- there is a line through the occupation space meaning they have none.
Interestingly, they are listed after all of the Russell children, but before an Elizabeth Russell, aged 38.This may be John Allen in the 1861 census:
Durham, Restigouche Co., New Brunswick
Allen, John, Head of Family, aged 28, b. NB, farmer, Roman Catholic Allen, Margaret, Wife, aged 23(?), b. NB Allen, Charles, Son, aged 3, b. NB Allen, John, Son, aged 3 weeks, b. NB
The surname seems to be spelled as Allain in the Restigouche Catholic records.