Well, the informant on old Williams is his son in law, William Clark. The husband of his daughter, Jean.
His daughter Grace, died in 1919, the informant on that death certificate is her son, William Douglas.
It may be worth noting that William was just 6 years old when his grandfather died, so he would likely have no memory of him. He would only know about him from stories, I would assume. Also as mentioned before, the mothers name on Graces death cert is Annie Campbell.
As for sons Hugh and Samuel, the informant on Hugh's death cert is his own son, John.
Samuel's informant was his wife, Mary.
I have noticed that on Hugh's death certificate in 1867, his mother is listed as Ann Riddick, maiden name unknown.
This may be a stretch, but is it possible that William was married previously to another woman called Ann, and then he was widowed with young children, then remarried to Ann Jeffs?
This may explain why Hugh was born in the early 1800's, at which point Ann would herself just be a child.