When I was a kid I liked
1) Comparing your chances of dying to your chances of being run over while crossing the street
2) Comparing the injury rate to a seemingly "safe" sport with a higher injury rate (ex. you are statistically more likely to be injured bowling than playing paintball).
Not sure about dirtbiking (i've certainly had a concussion from it, and almost drowned from getting knocked out while crashing in to a lake LOL), but I'm sure if you do some research there are some good statistics backing up its safety. Hell you can probably prove its good to give a 6 year old a gun if you spend enough time going through NRA articles so I'm sure you can pull it off.