Well, you know that Knut died sometime after the 1920 census & you know Olive
died sometime after the 1930 census.
I would try writing the Onawa Public Library in Onawa & they may just have
both of them in their obituary files. Their address & phone is listed on the
Monona County website.
Another angle would be to ask them if they have the estate records for Knut
or Olive & that should tell you who their children were and where they lived.
The genealogy section of the library has lots of wills and administration for
settlement of estate records.
Also could be something in the histories of Moorhead (Spring Valley Twp.) or
Willow or in the Monona County histories which the Onawa Library has.