What on earth for?
Teach the child to control the bicycle properly.
I cannot think of anything worse, someone else having control of the vehicle you are supposed to be in control of.

The bike may stop in 0.5m, but what happens to the child?

This is far more likely to end up with the rider crashing, they are wobbling along and someone without warnings brakes the rear wheel, the bike will hit the ground instantly, same if the kid turns to avoid a crash as the parent hits the brake. This should never be allowed to be sold.

This is brilliant. Kids of two on a balance bike just don't understand danger. We've had several near misses, and if you're already pushing a buggy, you can't run after them. Reins don't work, as they would pull them off the bike. This would keep them alive long enough to learn to control the bike properly.

Sounds like a bad idea to me.

I'm working on a one piece suit stuffed with cotton wool in my garage. Should stop 'little Jimmy' hurting himself.

turn unexpected braking = crash
wle

If you don't think this is a good ides don't buy it. I think this would be great for anyone who lives on hills.

Nifty idea- but even better is following your kid- I run beside the kids on a me-mover www.me-mover.com making it possible to safely following at any speed while beeing able to support the kids and if needed stop them :) /j

While we're at it, we should install remote door brakes on all the doors in the house to prevent any accidental slamming of tiny fingers, remote seat belt actuation so they won't have to remember to buckle up, remotely deployed face shield to prevent black eyes in dodge ball, remotely actuated knee pads to save those knees from...life, personal air bags, floatation aids, remote heimlich device, remote ice packs, remote hydration infusers...
Another excellent product aimed at the "look at me, I can connect to and control the entire universe on my smart phone but I don't want to spend the time to actually parent" crowd not dissimilar to the Huggies Tweet Pee diaper in technology overuse. Sure, the vigilant (er...hover) parent can prevent run-away crashes, but that assumes that the parent not only has the remote in hand ready to use but also be paying attention at every moment of the child's existence. But if you are vigilant, couldn't you just tell your child to stop before the requirement of an emergency remotely actuated brake becomes necessary?
The obvious solution is to educate the youngster about safety in the first place. He/she won't listen when you yell stop? Then take away their riding privilege. They learn pretty quickly. How hard is that?