I got a Tread a few weeks back and I'm a bit confused. A few things...
There is literally a clip that attaches to you where the motor stops immediately when it is moved with any pull whatsoever. I tried it and the thing stops no matter how fast you are going. So far, this feels safer or at least the same as the typical treadmills I've used in commercial gyms.
That key is supposed to / technically required to come off when not in use. Very simple, right in front of you type item. If I had small children (mine are older and I still won't let them on) this seems like a no brainer as you can simply pull it out, put it on the Tread shelf, and the thing won't turn on at all.
I'm not even sure how a small kid could turn it on as the knob is like 6 feet off the ground and inside the handle. To get the speed up you would actually have to turn multiple times. If a kid or dog happens to wander under the tread that seems like a problem with the user. The beast is like 500 pounds and really needs a dedicated space.
Lastly, in general all exercise equipment should be kept from kids. I have multiple thousands of pounds of equipment in the garage that could be considered death traps.