In the area where you live, do most children wear bicycle helmets when they ride their bikes? What are the streets/traffic like in your area? Does your state/county/town have helmet laws for kids?
Where I live (Maryland) we have a helmet law for kids 16 and under, but I rarely see kids wearing helmets when we ride in the neighborhood closest to us. I've never known anyone to get a ticket or a fine for allowing their kid to ride without a helmet. My H and I happen to live on a rural 2-lane road, so our kids cannot just go outside, grab their bikes, and pedal off to a neighbor's house. When we ride, we are going for a longer ride as a family activity, and we're likely to be on bike trails or busier roads. Because biking is a whole-family activity for us, not just something the kids do on a quiet cul-de-sac, we always wear helmets. If we lived in a neighborhood, I might not tell the kids to put helmets on every time.