Hallo Attie,
About Fokko and Mettje I think I have found the following (I will write it exactly how it is written in the records):
1900 census, West Town, Chicago City, Cook County, Illinois, USA
-Jager, Otto; born January 19, 1863, age 37, married 17 years, born in Holland, parents born in Holland, came to America in 1891, occupation of carpenter
-Jager, Mary; born February 6, 1861; age 39, married 17 years, born in Holland, parents born in Holland, came to America in 1891, no occupation
1910 census, Chicago City, County of Cook, Illinois, USA
-Jager, Otto; age 47, married once, married at age 25, born in Holland, parents born in Holland, immigrated in 1891, occupation of carpenter
-Jager, Mettey; age 50, married once, married at age 28, no children born, born in Holland, parents born in Holland, immigrated in 1891, no occupation
1920 census, Chicago City, County of Cook, Illinois, USA
-Jager, Otto; age 56, immigrated in 1891, born in Holland, parents born in Holland, occupation of carpenter
- Jager, Mettie; age 62, immigrated in 1891, born in Holland, parents born in Holland, no occupation
1930 census, Chicago City, County of Cook, Illinois, USA
-Jaeger, Otto; age 67, married at age 21, rents house for $15.00 per month, born in Holland, parents born in Holland, naturalized in 1893, occupation of carpenter
-Jaeger, Mette; age 70, married at age 24, born in Holland, parents born in Holland, naturalized in 1893, no occupation
There was a "Mattie Jager" who died on 28 May 1938 in Chicago but I am not sure that this is Mettje. I could not find anything else for Otto (Fokko).
So it appears that Fokko and Mettje had no children and died in Chicago, Illinois.
mvg, John