If you don't have it yet. The Puffer Genealogy is a book published in 1915. The author was Charles Nutt who used both his own research(he was a professional genealogist) and the research of Dr. Loring Puffer who gave him over 40 years of research.
Neither Joseph Foster or Mary Elizabeth Puffer's death is listed in the book.
I would check Rockville, CT and Stafford Springs Ct since they lived there. Ella is listed as living in Boston in 1915. It is possible she took care of them so you might try their also. Tracking the children is probably the best way.