Thanks for the suggestions. I do have DOBs and DODs for the family. Marion, the longest-lived of the Neillie children, died only a few years ago. Unfortunately, I hadn't begun my genealogical research until after her death. That's the way it goes. I did, however, contact the funeral home, and they agreed to which forward my letter to her nephew. (She never married and had no children.) It's possible that we made contact on Find A Grave, but I haven't gotten further responses.
I was able to obtain a rather blurry group photo from Gallaudet University Archives, and was told that one of the gents was probably Charles Neillie. So I was able to run the "Deaf Almanac" entry on him that I'd been wanting to do.
I'm planning to have an artist illustrate the biographical sketch I'd like to publish in DEAF LIFE, but I haven't given up the hope of connecting with the family. There ARE living descendants, yes.