There's a George W Johnson, b. 1839 in Canada East, in W. Hawkesbury on the 1861 census, tinsmith. He's a singleton, so if this is the correct George, he married Margaret in 1861 or 1862 and had Robert shortly after getting married.
Ontario, Canada, Marriages, 1801-1928 about George Johnson
Name: George Johnson
Birth Place: North Gore, Quebec, Canada
Residence: Vankleek Hill
Estimated Birth Year: abt 1838
Father Name: Alexander Johnson
Mother Name: Rachel Johnson
Spouse Name: Margaret A. Flynn
Spouse's Age: 19
Spouse Birth Year: abt 1843
Spouse Birth Place: Vankleek Hill
Spouse Residence: Vankleek Hill
Spouse Father Name: Benjamin Dexter Flynn
Spouse Mother Name: Margaret Flynn
Marriage Date: 21 Dec 1862
Marriage Location: L'Orignal
Marriage County or District: Prescott
In the 1871 census (W. Hawkesbury), George is 33, Margaret Ann is 27, Robert G is 8 and Alice Mary is 6. The information all fits together with the marriage and the previous census.
Unfortunately there's nothing on either George or Alexander in the Prescott/Argenteuil history book.
In the 1851 census, in Gore (Deux-Montagnes Co, QC) There's an Alexandre Johnston, b. 1798 Ireland, wife Rachel, and a number of children. If you don't have access to the 1851 census here on Ancestry, it's free at automatedgenealogy.com