Your family is tough. I had a terrible time finding any census information. I have to assume that they are either mis-indexed in 1900 or the name recorded might be Johnson (as given in the marriage record for William and Nellie).
Here is an 1880 census possibility for Sophia
"United States Census, 1880," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MDL4-NTV : 14 July 2016), Sophia Busch in household of Charles Busch, Muscatine, Muscatine, Iowa, United States; citing enumeration district ED 248, sheet 198C, NARA microfilm publication T9 (Washington D.C.: National Archives and Records Administration, n.d.), roll 0358; FHL microfilm 1,254,358.
Here is your great-grandfather in 1920 census:
"United States Census, 1920," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M81J-67G : 14 December 2015), William Youngton, Cedar Rapids Ward 4, Linn, Iowa, United States; citing sheet 6B, NARA microfilm publication T625 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 1,820,500.
There are four marriages (on the same date) recorded for Theodore and Sophia: with these spellings for Theodore: Theodore C Youngturn, Theodore Younghim, Theodore C Youngton, & Theodore C Youngtum. The one gives you Sophia's parents named Charles Bush and Mary.
"Iowa, County Marriages, 1838-1934", database, FamilySearch (https://familysearch.org/ark:/61903/1:1:KLWZ-JQ6 : 8 March 2016), Theodore C Youngtum and Sophie M Wilhelmire Bush, 1883. -- parents are Charles Bush and Mary; Sophie is born in 1861.
There are a few Youngtons in Iowa at Findagrave, none of the older people though.