Found no DC on file in MIssouri.
Middle name from son, Elmer's DC.

Robert appears on these two Censuses with his parents:
1850/ Kentucky/ Hart
James Grinstead, 37y b.KY, occ: Farmer
Eliza, 28y b.KY
Thomas, 9y b.KY
Robert H, 7y b.KY
Amanda J, 5y b.KY
Albert L, 3y b.KY
Mary E, 5mos b.KY
plus one other

1860/ Missouri/ Harrison
James Grinstead, 45y b.KY
Eliza, 37y b.KY
Thomas B, 18y b.KY
Robert H, 17y b.KY
Amanda J, 14y b.KY
Albert L, 12y b.KY
Mary E, 10y b.KY
Martha F, 8y b.MO
Sirilda S (f), 5y b.MO
John W, 3y b.MO

Detailed Soldier Record
GRINSTEAD, ROBERT H
Battle Unit Name: 23rd Regiment, Missouri Infantry
Side: Union
Company: D
Soldier's rank in:  Private
Soldier's rank out: Private
Alternate Name: blank
Notes: none
Film Number M390 ROLL 18
http://www.nps.gov/civilwar

He appears on the 1870 Census with his father-in-law, Henry O'Bannon Neville.

1880/ Missouri/ Harrison
h/ Robt H Grinstead, 37y b.KY, occ: Hardware Merchant
w/ Martha H, 31y b.KY
s/ Elmer E, 12y b.MO
s/ Harvey D, 7y b.MO
residence: Eagleville

1890 Surviving Soldiers/ Missouri/ Harrison
name: Robert H Grinstead
rank: Private
company: D
regiment: 23 Mo Inf
date of enlistment: 10 August 1862
date of discharge: 10 June 1866
length of service: 2y 1m
PO address: Blythedale, Harrison County, Missouri
disability: Eye and Kidney trouble
remarks: Severe spell Typhoid fever ??

1900/ Missouri/ Harrison
h/ Robert H Grinstead, 57y b.Jun 1843 KY, M/34, occupation: ?Deals Livestock
w/ Martha A, 52y b.May 1848 KY, mother of 3/2 living
fil/ Henry O Neville, 84y b.June 1815 KY, Wd

1910/ Kansas/ Wyandotte
h/ Robert H Grinstead, 64y b.KY, M1/43, occ: Stockman
w/ Martha, 60y b.KY, mother of 3/2 living
residence: 1225 Cleveland Avenue, KCK

1920/ Oklahoma/ Osage (Census dated 10 January)
h/ Robert H Grinstead, 76y b.KY, no occ
w/ Martha, 71y b.KY
residence: 315 13th Street, Pawhuska

Link to son, Harvey Dean Grinstead. Sent edit to link parents 08 June 2021.

Links to sisters, Amanda J (Grinstead) Dale and Sirilda S Grinstead. Sent edits to link parents 08 June 2021.