I once knew a lady who lost her daughter in a car accident. Not the same thing, obviously, but a similar kind of pain. The way she remembered her daughter was to wear a pin with her daughter's picture on it to all family gatherings, etc. That way she said her daughter got to be there with everyone as well.
Would something like that work? Do you have any pictures of Katelyn you could use? You could wear the pin yourself or give it to the parents to wear. I would still do as Somer said and run it by your son first, privately, to get his take on how well they are coping.
Additionally, though, my step-grandparents lost a son in a car accident. When I first met them, I was told to not ever bring it up in front of them because they still were not able to talk about it, even though years had passed. And more years went by before I ever did hear them talk about him, or especially the car accident he died in, but eventually they were able to talk about it. So I'm mentioning this to reinforce the suggestion to run anything you want to do by your son first, to avoid any catastrophes.
