There are a couple of things that people are told in relation to bike helmets. I am concerned with the third.
The first is that everyone should wear a helmet whenever riding a bicycle (not: kids have it placed as a safety expectation, but one that clearly doesn't apply to parents).
The second is that if you get in an accident and fall on your helmet, congratulations! You've destroyed completely the value of a perfectly good helmet, instead of the person suffering possible brain injury. Throw it away and buy a new one that will protect for the next accident.
But the third is that there is a right way and a wrong way to put on a helmet, and the wrong way has a majority share.
The right way is illustrated in the first picture, and the wrong, more common way, is in the second picture:
The difference, beside the fact that the right way looks dorky, uncool and way too far forward, is that if the person portrayed in the first picture were to faceplant hard, the helmet will take a good chunk of the impact; it may not directly protect the whole face but in a sizable portion of bicycle crashes the helmet will significantly mitigate the force of the impact.
The second way that seems obvious and looks right will leave plenty of possibility for the helmet to be pushed out of the way, or not need to be pushed out of the way, so that if the person faceplants the helmet may have little effect beyond the ornamental. Having a helmet worn this way is a bit like wearing a seatbelt that is halfway to being pushed far in enough to click, and I have seen this way of wearing a good helmet on the picture on the front of a medical group's safety brochure explaining to parents why children should wear helmets for certain activities.
So what is the best response when I see parents using helmets in some sense but not in a way that will really help in an accident. Mostly I've kept quiet when this happens, and I originally looked for a manners or communication StackExchange site to post this in, but I'm wondering what is an appropriate response when they think buying a helmet is enough, and do not seem to recognize that there is more to getting serious benefit from a helmet than somehow having the helmet touching a child's head.
So what kinds of responses are helpful when parents have paid for a helmet but aren't using it nearly as helpfully as it could be used?