(these sprang to mind)
At least you have said your bit now, if one of them does get into an accident at least you know you did what you could to warn them of the dangers.
Unfortunately - like so many things, it takes a tragedy before people realise the error of their ways.
Maybe they will stop and think just for a moment about what you said before going out helmet less next time - but chances are it still wont stop them.
Well done for making an effort to try and help.
Maybe get a Road Safety Officer to pop by and have a word.