I'm the one who requested the photos of their headstones (yeah, I know, I have a strange user name on Find A Grave). I found the details on their graves from the Gravesite Search web page. It said they were in section Park E. If William's headstone is in poor shape, it's entirely possible their headstones either sunk down; or they are so badly eroded as to be unreadable. Another possibility is they were exhumed and their coffins put in the same grave with their father. This was actually done with a sibling who died as an infant; her maternal grandmother's grave was opened and her casket placed on top. There is no mention of her on the headstone as of now, although my Mom would like to place a small marker for her.