In Robley Evan's 2nd autobiography, "An Admiral's Log", he writes that after having given up command of the Atlantic Squadron in San Francisco due to illness, members of his family came to escort him back east.  During the train ride back home his family informed him that his "name sake grandson" had died a week or so before and that the news had been kept from him because of his condition. Unfortunately I don't have the book in front of me know but I think Evans did identify the deceased as being the son of the Seawalls. I think this information in "An Admiral's Log" supports you idea that the deceased was Evans' grandson, and would place his death in 1908.
Regards, Chris Rasmussen.

About a year ago I e-mailed you regarding Robley Evans Sewall.  I attached paragraph from Admiral Evan's book dealing with his grandson's death which I  have been meaning to provide to you for some time. I have also attached a photo of the  admiral and his grandson.
Chris Rasmussen

"When the train had started from Oakland I was told of the death of my only grandson, Robley Evans Sewall, a son of my daughter Virginia, wife of Mr. Harold Sewall of Boston.

"My namesake had been dead a week, and yet not a word or intimation of the sad news had been allowed to reach me because of the work I had to do and for fear of its effect on me.

"How those about me managed to keep every mention of the child's death out of the newspapers, and above all how they kept smiling faces for me when their hearts were breaking, may not be told here. When the cruel blow came the blue all came out of the sky, and it was my turn to face with a smile the crowds that cheered me on my way.

"That the precious young life around which so many hopes centered should be taken is one of those mysteries which my be solved in the world to come; certainly it never can be in this."

R.D. Evans, "An Admirals Log". Pages 451-452.

Photo By M. R. Patterson, October 2002