Since Susie and son Percy are living in the same house in the 1900 census with a Mims family, have you looked for a Stevens-Mims marriage or other connection?
Don't forget to look at listings with the alternate spelling of Stephens.
And don't overlook the possibility the "Wd" was sometimes used to designate divorced or abandoned women, in an era when divorce was rather scandalous. Or even that Stevens might be Susie's own name. I see she is listed as an "authoress" in the census. That would have been somewhat Bohemian in 1900.
Have you found a death record for Susie? I see that Princton died in California. If Susie died there as well, California death records quite often have the surname of both parents, even in the index.