The Genealogical Record of the Barnum Family confuses Robert Grandison Barnum with his 5th cousin, Grandison Ansil Barnum.
In the 1900 US Census for District 404, Brooklyn Ward 3, Kings County, New York Robert G. Barnum, 25, was enumerated as a boarder with the family of Charles Peck.
In the 1910 US Census for District 1420, Manhattan Ward 12, New York, New York (Clermont Avenue) the family of Robert G. Barnum was enumerated as one of 16 families boarding in the home of Ernest G. Sicard, as follows:
Dwelling #8; Family #92
Barnum, Robert G.; Head; M; W; 35; 1st marriage; Married 1 year; b. Connecticut; Both parents b. Connecticut; Speaks English; Salesman; Hat Company; Wage earner; Employed full census year; Can read and write; Renter
Barnum, Myrta R.; Wife; F; W; 33; 1st marriage; Married 1 year; b. Ohio; Both parents b. Ohio; Speaks English; No occupation; Can read and write
Barnum, Russell P.; Son; M; W; 2/12; Single; b. New York; Father b. Connecticut; Mother b. Ohio; No occupation
In the censuses of 1920 and 1930, Robert, Myrta and their three sons were enumerated in Montclair, Essex County, NJ. Robert Jr. was born about 1911 in New Jersey and Donald C. was born about 1916 in New Jersey.
World War I Draft Registration Cards, 1917-1918. Name: Robert Grandison Barnum; City: Upper Montclair; County: Essex; State: New Jersey; Birth Date: 22 Jan 1875; Race: White; Roll: 1712102; DraftBoard: 3; Age: 43; Occupation: Salesman; Nearest Relative: Myrta R. Barnum; Height/Build: Tall/Slender; Color of Eyes/Hair: Brown/Gray
From The New York Times, May 28, 1937: Died-At his home, Upper Montclair, New Jersey on Thursday, May 27, 1937, Robert G., husband of Myrta Russell Barnum and father of Russell P., Robert G. Jr., and Donald C. Barnum.