There is a Melvin Johnson, son of Harry and Amy, with a sister Marjorie in the census data, (along with the death cert. listing parent which you seem to have already found). Not sure if this is much help.
Illinois, County Marriages, 1810-1934
Citing this Record: "Illinois, County Marriages, 1810-1934," index, FamilySearch (https://familysearch.org/pal:/MM9.1.1/KF26-58V
Name: Harry Johnson
Marriage 16 Nov 1912 Kane, IL
Age: 21 Birth (Est) 1891
Father Chas Johnson Mother Lena Benson
Spouse Amy Klingberg
Age: 20 Birth (Est) 1892
Father Sanfred Klingberg Mother Annie Johnson
U S Census, 1930 Chicago (Districts 0501-0750), Cook, IL
Citing this Record: United States Census, 1930," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/XSLJ-XLJ
Head Harry Johnson M 38 IL
Wife Amy Johnson F 38 IL
Son Malvin Johnson M 18 IL
Dau Margorie Johnson F 16 IL
U S Census, 1920 Kankakee Ward 1, Kankakee, IL
Citing this Record: "United States Census, 1920," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MJ4F-Z53
Head Harry Johnson M 28 IL
Wife Amy Johnson F 26 IL
Son Melvin Johnson M 6 IL
Dau Marjorie Johnson F 5 IL