Christine, it doesn't sound as though you've done your census work yet, since Herman was already in the US for several years by 1910. Starting with the latest available census and working back:
— 1930 census: Herman (age 49, wid); Roy (19) and Ruud (16) are in Seaside (Dist 37), Clatsop, OR. Herman is listed as being an alien.
— 1920 census: Herman (indexed "Mamrila"; alternate spelling Mamila, Mamula) in Seaside, Clatsop Co., OR, age 39; widower, immigrated 1901. Roy is 9 and "Ruut" as it's listed, is 6. So Milga d. sometime between Ruud's birth around 1914, and the 1920 census. In the column "Naturalized or alien" he put "Pa" (meaning first papers were filed. Look for these first papers. Even if he did not finish the naturalization process, the Declaration of Intention may tell you where in Finland he was from.)
— Herman Mannila World War I Draft Registration shows residence Seaside, Clatsop, Ore.; DOB 24 April 1880; nearest relative Milga Mannila. There is no date listed but since those records are from 1917-18, Milga's death can be narrowed down to ca 1917-1920**(see at bottom of this message). Herman is listed as a declared alien; not a US citizen.
— 1910 border crossing lists Herman as married, a farmer, going to Portland, Ore. This sounds as though he made a trip back to Finland, since the 1910 census below indicates he was already in the US for several years. Town of origin looks like Jalasjawi? I cannot read the name of the relative in Finland.
— 1910 census: Herman (age 29) and Milga (age 27) Mannila, Seaside Precinct, Clatsop, OR; married 3 years (wedding therefore ca 1906-07 and likely in the US; perhaps OR). Herman is listed as immigrating 1901; Milga in 1902.
— Oregon Death Index 1903-1998 did not show Milga Mannila. She was in Seaside for many years, as was Herman after she died; so it seems logical that she died near there. Checking for alternate spellings or handwriting quirks, I put into the search box only the first 3 letters of her first and last names. This is likely her:
Name: Mannita, Milka
Death Date: 22 Oct 1918