When I was with my ex-girlfriend her little sister was knocked off her bike, also by a totally reckless driver doing over 80km/h in a 50km/h zone. She suffered serious head trauma, they didn't actually think she'd make it through the night. She did survive but had pretty serious behavioural problems after that, and died a few years later of a cerebral tumour which was almost certainly related.
I'll never forget the surgeon when he first came out of the operating room. He said: "If she had been wearing a helmet, none of this would have been necessary and she'd be fine." You could see clearly in his face that he'd seen this way too many times.
I have another more recent anecdote since having kids myself. A friend of a friend here has a son the same age as our daughter. When he was 3 he was out on his scooter without a helmet, and fell off - nothing out of the ordinary, except he happened to hit his head on the concrete corner of a ramp. We live in a small town, he had to be choppered out to the nearest city, spent significant time in ICU, multiple brain surgeries, and the kid is now epileptic.
I've also had the experience of falling when snowboarding not wearing a helmet, and not even going very fast. I woke up inside an MRI machine several hours later after a serious concussion.
You can do what you want for yourself and your kids. Mine are going to wear helmets. There is no learning curve for head injuries.