I get where you are coming from, but you want to associate bike riding = helmet wearing.
From training wheels to full-speed downhill riding should only take 6 months. You want to encourage helmet wearing as a habit before that progression occurs.
My child had a very large crash that occured because he went down a hill and got confused as to which side of the bike the brake was on, because he was riding an unfamiliar bike. He had ridden down that hill many times. He crashed heavily at a high speed, enough to knock a tooth out and lost a lot of skin on his face, elbows, hands and knees. The helmet had serious damage and had to be thrown away. I don't want to think what would have happened had there been no helmet. He was 4 at the time.
I let them ride down hills to get used to the speed as part of learning, so that when they do get older they don't make bad mistakes. I also ride on the road with them to develop their traffic awareness. But helmets are a vital part of that.
I had several high speed crashes on my bike during school years. It was only luck that meant I didn't hit my head hard.