I found Charles J Brooks in the 1920 census Cook County, Illinois, City of Chicago 35th Ward. Series: T625Roll: 357Page: 271.In his household were
Charles J Brooks, head, 27 years old, male, white, Born in Illinois.His father was born in Scotland and his mother was born in Canada.His profession is listed as a Chauffer.
Jennie Brooks (Must be your Jennie Jansen) white,28 years old, female, white, born in Michigan parents are both listed as Holland Dutch without profession
Charles J Brooks, son, 8 years old, Male, white, Born in Illinois, without profession
Jacob Brooks, son, 5 years old, Male, White, Born in Illinoiswithout profession.
I couldn't find any thing on Jacob Jansen and Mary.I looked in Both Illinois and Michigan censuses.All of the Jacobs I found had wives with names other than Mary. However, in the Oakwood Cemetery in the city of Muskegon Michigan I found in the burial records:
Jacob JansenGrave Description 3-19-12 Death date November 12, 1927
Marie JansenGrave Description 3-19-12 Death Date December 29, 1904
Do you recon that these may be her parents.I would provide these dates to the Muskegon Genealogical Society and see if they may be able to pull obituaries on these two, or try Random Acts of Genealogical Kindness at www.raogk.orgI would pull them for you if I could, but I live in Texas.I hope that this helps!!