You need to search http://pilot.familysearch.org/http://pilot.familysearch.org/. There you will find the marriage of Elen ALTMAN and Seba W. HART on 8 Mar 1899 in Hancock Co. OH.
You will also find his second marriage in Hancock Co. on 9 Mar 1912 to Anna B. HAMMON.
You can read Sarah Ellen's death record there, too, but to see the image of her death cert you need to register first at http://fsbeta.familysearch.org/http://fsbeta.familysearch.org/.
In the 1920 census the HARTs are in Sapulpa, Creek, OK: head, S. W., 45, OH OH PA, driller-oil fields wife, Anna, 38, OH OH Germany, housewife son, Darril, 16, OH OH OH, glass plant son, Ivan, 12, OH OH OH
I did not find a death of the "week old baby" in OH. Perhaps, he/she died in or enroute to OK.
In 1930, only Seba and Anna are in Detroit, Wayne, MI but Ivan may have married Edna and they were in Riverview, Wayne, MI. I don't know of any Michigan marriage info online after 1925 nor MI deaths online 1921-1970.
However, the MIDI shows an Ivan A. HART b 16 Jul 1908, d 31 Oct 1983 and an Edna HART, b 29 Apr 1911, d 20 Sep 1987. Both died in Briley (Township), Montmorency, MI.
Check for the library in Atlanta, MI at http://montmorencylibrary.com/http://montmorencylibrary.com/ from which you can request copies of Ivan and Edna's obit.
I am not related.
PS. Seba's middle name was Wayland per his WWI draft registration in Creek Co. OK.