Is this your William Herschel?https://familysearch.org/pal:/MM9.1.1/K7V6-GJH
This looks like them in 1900.
Jefferson Township, Cashton Village, Monroe County, Wisconsin
James W. McMullen, age 31, born Wisconsin, parents bp unknown, Drayman
Ella V. McMullen, age 28, born Wisconsin
William McMullen, age 7, born Wisconsin
Harold C. McMullen, age 5, born Wisconsin
Blanche McMullen, age 4, born Wisconsin
Arlie McMullen, age 1, born Wisconsin.
If you don't have the marriage record for James and Ella, I would suggest you order it here - it may tell his parentshttp://www.wisconsinhistory.org/vitalrecords/index.asp
They married June 4, 1890 in Vernon County, Wisconsin - she was listed as Emma Jane Maxwell
I would suggest you post a message on the Monroe County, Wisconsin board, as that is where they lived. Also are you familiar with this index?http://www.monroecountyhistory.org/