As luck would have it, I found everything else other than the marriage records. This is a farming family, so they didn't move a whole lot. In the 1860 census for Cincinnati OH there is a George SENTHOFF, 32, b Holland, wife Ellen, 25, b Oldenburgh, mother? Sarah, 59, b Holland.
In the 1930 census for Starr, Hutchinson Co, SD:
George SANDHOFF, 65, b OH
Annie, 55, b NE
Joseph, 24, b NE
Mary, 22, b NE
Helen C, 20, b NE
Frank, 19, b NE
(Anton) Anthony J, 17, b SD
John, 15, b SD
Andrew, 13, b SD
Other sources show various records with:
Joseph - b 23 Jul 1905, d Oct 1981, Dimock SD
Mary - b 21 Dec 1907, d 27 Nov 1994, Dimock SD
Helen C - b abt 1910, M. 26 Apr 1932 - Arthur Hohn, SD
Frank - b abt 1911, M. 14 Jan 1936 - Loretta Roeder, SD
Anton - b abt 1913, M. 27 Nov 1945 - Louise Bowar, SD
John - b 3 Aug 1914, d Nov 1986, Dimock SD
Andrew - b 19 Nov 1916, d 18 Sep 2002, Dimock SD