I don't use anything. If they can't examine a knife without cutting themself, they probably shouldn't be handling them to begin with. I usually just say "these are seriously sharp, if you run your thumb down the edge to see how sharp it is, you will get blood all over the knife and ruin it, so please don't."
Some people just can't resist putting the edge to their skin for some reason, and need to get cut in order to learn, IMHO.
You could probably just go to home depot and get some small diameter rubber tubing and slice it lengthwise, if you wanted to....