I think you're getting some of the relationships a bit messed up. From what I found Grace was Raymond's wife and Reye and Charles were their children.
The California Death Index shows Mary died in San Diego as did Raymond Gilbert Keenan (July 11, 1954), Bertha May Allen (Dec 20, 1969) and Grace Keenan (6 Oct 1888-20 Dec 1966). The census shows Mary only had two children.
The 1900 census shows the widowed Mary (misindexed as Mabel, to be fair it is hard to read) living Lincoln Twp, Kearney Co., NE. She was born Aug 1868 in Iowa. Bertha and Raymond's father is listed as born in Ohio. Bertha was born in Nebraska and Raymond in California.
Raymond's WWI draft card shows he was born in San Diego.
Some of these records you can look at for free at labs.familysearch.org and Ancestry.com. For those you cannot see for free I suggest checking with your local libraries to see if they have Ancestry or Heritage Quest.
I would imagine Charles is buried with Mary. Aside from any tombstone data the cemetery office may have something in their records that might be helpful if only an age at death. I would imagine his death was reported in a local newspaper. It might have at least some helpful data. The church he was a member of may have something in their records that could be of help. I don't know what kind of records California or San Diego kept back in 1895, but it would seem worth looking into.