The Social Security Death Index shows Florence born 30 July 1907.
The 1910 census shows Florence Kerstetter, age 2, as the daughter of William H. Kerstetter, age 35, and Elizabeth, age 28, married about 4 years living in Chapman Twp., Snyder Co., PA. Oddly Amanda Glace, age 73, is shown as a boarder rather than as his mother (read on)
The 1920 census shows Florence, age 12, as the niece of Oliver and Emma Renninger, living in Washington Twp., Snyder County.
The 1920 census also shows William & Elizabeth with their daughter Margaret, age 6, living in Susquehanna Twp., Juniata County. Why Florence wasn't living with her parents is a good question.
The 1900 census shows William, born Nov 1874, as the son of Amanda Glace, who was born June 1837 and widowed.
The 1880 census shows William and Amanda, along with Frederick Glace as the step-children of George Stephen and his wife Margaret.
A WWI draft registration shows a William Henry Kerstetter, born 7 Nov 1874, living in Richfield, Juniata County. Be careful. There were several men with the same exact name born around the same time.
You can look at some of these records for free at labs.familysearch.org and Ancestry.com. for those you cannot see for free check with your local libraries to see if they have Ancestry or Heritage Quest.
Check with the Snyder County and Juniata County Historical Societies to see what they may have.
I suggest getting Florence's birth certificate from the PA Div. of Vital Records. Since you are a direct descendant you should be able to get it. Include her date of death to help the process. You could also try to get her death certificate from the same office. either one should have the mother's maiden name.
Once you know when William died try to get his death certificate and obituary. Both would be helpful in knowing who his father was and the mother's maiden name. You can also try to get Amanda's DC once you know when she died which was probably in the 1910's.
Since you know William was married around 1906 see if the Snyder County Courthouse Register of Wills has the marriage record (assuming they married in Snyder County). It might have info on his parents.