I need a key to start my Sole, and you can attach the key to yourself while operating it (in case you fall off). A child getting mangled under the tread couldn’t have happened with that simple mechanism in place.
Unless supervision means the child never gets further away from the supervising adult's arm's reach, I think the only thing you can do is fence off the treadmill to keep free range kids and pets away.
Doesn’t seem likely. His parent would either fall because the treadmill slowed down, running into him and injuring them both or they would have told them to stop it because it’s clearly dangerous