I'm not much of a gravesite visitor myself. When my dad passed away I knew that he/his spirit was in heaven and it was just his body that was in the ground. So I can probably count on both hands how many times I been to the graveyard, and those times were just to take my mom to visit. I don't get any kind of peace or comfort by looking at a plot of dirt, and I was very close to my dad. My mom gets mad at me for not going, but she is very old-school about those kind of things.
But I think the whole picture taking thing is really up to the individual though. And if the kids in the OP didn't have a problem getting their picture taken at their mothers grave, then fine. It's probably not something I would do though.
But.... the whole taking pictures of dead people in their caskets... well that definitely creeps me out. Probably because when I was about 13-14 they did this at my grandmothers funeral. Too weird for me. Like others have said I would rather remember that person alive.