This predates civil registration in Ontario by several years (began in 1869). Have you checked with the Anglican Archdiocese of Toronto for St. Margaret's records which may have survived. However, even a burial record would not give her maiden name. Baptisms for the children may be your best source.
The 1861 census shows Henry as a widower, with no death listed in 1860, so Sarah died before 1860. The children are Charlotte, aged 19, Mary Ann, aged 15, Elizabeth, aged 14, James, aged 10, and Sarah, aged 8.