Successfully spreading the legend would keep people from playing with laser pointers

I doubt it. Urban legends focusing on socially or at least parentally discouraged behavior are pretty common; witness the popular and long-lived one about the killer targeting teenagers who have sex with each other in their cars. They don't seem to deter many people, though.

Remember, by hypothesis we have kids with the resources to get dangerously powerful lasers and the will to use them. These aren't five-year-olds that can be cowed into good behavior by spinning a tale about the alligators that'll eat you if you go outside after dark; indeed, I didn't find that entirely convincing when I was five. You might even get people trying to match the legend's conditions just to see what'll happen; show of hands, who here tried to invoke Bloody Mary as a child by standing in front of a dark mirror and chanting her name?