What? Why would a kid do that? I’d ask why they did it and explain to him or her that it’s very dangerous and make them put the brakes back. If they seemed to comprehend why it was wrong to do then that’s that. If they are clueless then maybe there would need to be some sort of punishment I’m not sure though. It really depends on the kids reaction and explanation.

I would probably tell the kid they can’t ride the bike until they make it safe by putting the brakes back on.

Depending on the age of the kid, they may not understand fully what happens when you’re on a bike going really fast, maybe toward a crossroad where you need to brake, or toward a bunch of people, and you need to be able to stop. They may be thinking it’s going to be a lot of fun and reckless to be on a bike that just goes and goes without stopping.

If I had a spare lock, I’d secure the bike until I found out why they removed the brakes.

If their reason seemed like they weren’t intending to do anything dangerous, then I’d have them replace the brakes before I let them ride again.

Some examples of those reasons would be: they were center-pull brakes and they wanted to replace them with side pulls.
They noticed that one of calipers was fractured or seriously bent, and wanted to install new brakes.

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.

I would feel like I somehow failed basic parenting. I would think by the age my child is old enough to ride a bike (and remove brakes) that I would have taught them enough basic common sense for them to deduce why brakes are an essential element on any vehicle.

The only other possibility I can think of is that they might be engaging in intentionally self-destructive behavior as @Jeruba mentions. That would be it’s own set of serious concerns.

As I child of 7 years my brakes failed going down a hill. There was a busy intersection at the bottom so I stuck my foot in the spokes and I went over the handle bars. A friendly neighbour helped with my injuries.

Even in (very) young children, depression can occur.
To rule it out, I would visit the doctor with them, and have them examined for that.
Just in case.
I’ve known a ten year old who didnt want to live no longer.
He’s doing good now.

I did that when I was a kid. It was an experiment to see how good a bike rider I was. You can actually maneuver (through turns) to slow yourself down=.

I proved to myself that I could do it, and I put the brakes back on.

Of course, I lived on a very short and barely trafficked street, so it was safe. I wouldn’t have done it on a street that had a lot of cars.

I would start by asking them why they removed the brakes. You then might have an idea of where to go from there.

Have you asked this question? I can think of a few good solid reasons. I’d be interested to hear their answer.

At a spot along our greenbelt, there is a bicycle repair station with tools. Several times I have passed this spot to see brake parts, calipers, cables, and brake shoes/pads strewn about.

You could show them this video, but they might just dismiss it as if it’ll probably never happen to them.

I have an idea that @kritiper is talking about somebody else’s kid.