I might tell the kid a story. Here’s one.
When my son was 15 he and his girlfriend went for a bike ride in her hilly neighborhood. She’d ridden there all her life. She was in front, leading the way, when somehow she lost control of her bike and couldn’t stop it.
She was tearing down the hill at speed, screaming, heading straight for a busy intersection.
My son, right behind her, sized up the situation in a second, jumped on the pedals, and swerved in front of her at practically the last second, so she crashed into him. And stopped.
A shop owner saw the whole thing and called an ambulance. They wound up in the ER together.
My son was ashamed and contrite for having caused her collision and injured her. Her parents said, “Are you nuts? You saved her life! You’re a hero.” We were very proud of him for his quick thought and his taking a hard hit to save her. She’d have been a goner if she’d flown into that intersection.
They were both pretty banged up, but they were okay. He wore a scar over one eye for years.
Without any brakes, every corner could be a risk like that, hill or not.
I’d also deny the child use of the bike, any bike, until I was sure he or she understood clearly why you can’t kill the brakes. And maybe not then.
And I’d be watching, I must add, for any other sign of risk-taking or self-destructive behavior. Foolishness is one thing, and a pattern is another.