If you send for their death certificates from the Minnesota Historical society, that may give more information. Exact date of birth, (or exact age in years months and days, and you do the math) and even the place of birth and father's name.They only cost about $9 apiece, but sometimes are so helpful.
And Hulda's listing is this: (although the actual death certificate you send for will have the date of birth, place of birth and the mother's maiden name, sometimes the father's name.The index doesn't have it listed, but the certificate itself should.
JOHNSON, HULDA AUGUSTA Date of Birth:na Place of Birth:na Mother Maiden Name:na certid# 1918-MN-010599 Date of Death:26 Oct 1918 County of Death: MORRISON
JOHNSON, JOHN L Date of Birth:na Place of Birth:na Mother Maiden Name:na certid# 1908-MN-007486 Date of Death:22 Sep 1908 County of Death: MORRISON
In Sweden John L. Johnson could be John, or Johan, or Johannes, or Jon.John Johnson was usually an Americanized version.
on the 1910 census, in Morrison County, Minnesota, there is a John and Hulda Johnson, with a son Chester, born in Illinois, age 11, Chester is listed as the step-son, so did the first John Johnson die, and Hulda remarried another John Johnson?
Name: Chester Johnson Birthplace: Illinois Relationship to Head of Household: Step Son Residence: Pike Creek, Morrison, Minnesota Marital Status: Single Race : White Gender: Male Immigration Year: Father's Birthplace: Sweden Mother's Birthplace: Sweden Family Number: 19 Page Number: 2 Household Gender Age Birthplace SELFJohn A JohnsonM 56y Sweden WIFEHulda JohnsonF 31y Sweden Chester JohnsonM 11y Illinois Arthur JohnsonM 9y Illinois Helga JohnsonF 7y Illinois Hilding JohnsonM 5y Minnesota Gunnar JohnsonM 1y 2m Minnesota
That 1910 census, if you click on that link I provided above, also shows that Hulda immigrated in 1896. All the children are step-children of John A. Johnson.
Since the last two children were born in Minnesota, and the birth index is also online, you can send for their birth certificates, which should tell something.