Bike Helmet - YES!
They were not available in 50's and 60's when I learned to ride as a child and I did not wear one UNTIL I had CHILDREN in the 1980.s.
How else can you encourage them unless you set an example. I have always worn one since, city or country, cruising or riding hard.
I also wear a helmet when I downhill ski and ride motorized recreational vehicles.
Each person thinks it wouldn't happen to them, but rather it is only a hope it will not happen to them.
For those that do see the point, it only takes that one moment when your head hits the ground and end up with an ABI - acquired brain injury.
I have cracked one helmet, no injury. Unfortunately the broken leg(hip) and separated shoulder were more serious so my wife would prefer full body protection.