Sigh, you all do know there is a huge thread on A & S on this subject?
Do you know that in nations where they actually think that bikes are transportation, most people don't wear helmets? We don't wear them riding horses either. (if that made no sense - I live in a rural part of a third world nation, and yes we do use horses for transportation) In all our time in this area I have only heard to two significant accidents with bikes. One was when a drunk road off the bridge into the river and drowned, and another, just a week ago, who was killed while riding at night with no light by a motorcycle. (and the fault was deemed to be the motorcycle since no one has lights here except me)
But, if you are racing, or engaged in sports, you are pushing the edge of your ability and probably will fall out of control - hence a helmet makes sense. Also, if you don't ride that often, it also makes sense.
But, if you are going to make the argument that I just might fall, well, I am 50 years old and I assure you I fall a lot more walking than I do riding, and the distance to the ground is a lot more. My solution is to go slower when riding, well within my abilities. Been working for more than 40 years now. Not so well on my walking.
If you want a good causal effect, look into speed and doing stupid things. Speed kills - and no helmet will change that. You would do much, much better trying to convince people not to go 50 MPH down a hill, that would make a big difference, but I notice that rarely is that mentioned and if someone does say they went really fast down hill, you don't hear people saying "what!?, are you wanting to be an organ donor?!" But be an old geezer just pedaling around and people think that you are asking for it.
Generally speaking, people aren't very good at accessing risk. Riding without a helmet isn't particularly risky. Some of the activities that you do that make you want to have a helmet, are. You might want to think about the activities - or perhaps just go with bubble wrap.
Kids should have helmets on because they are learning and because many kids do risky things with their bikes. (I did)