Grandpa's name was Robert Hardin, and indeed he appears as an early apprentice to Wright in his Chicago Prairie House phase in the late 1800s/earliest 1900s.
Hardin is also mentioned in one of Wright's articles, In the Cause of Architecture, along with Marion Mahony, William Drummond, Francis Byrne, Isabel Roberts, George Willis, Walter Burley Griffin, Andrew Willatzen, Harry Robinson, Charles E White, Jr, Erwin Barglebaugh, and Albert McArthur (he of the Biltmore Hotel in Phoenix).
Pawpaw gets a mention in a letter from Charles E. White Jr. to “Walter” send on March 4, 1906:
“Hardin is a new man, who left the University of Illinois after one year. His tuition was paid by a fellow townsman in Texas, who has thrown him over since he has become so “degraded” as to go into Wright’s.”
The article and photo below appears in the neighborhood newsletter:
And here are some shots from Google streetview:
I don't find any other footprint of Mr. Hardin on the interwebs, so I'm not aware of any other structures he designed.