In the video that another user posted the treadmill was already running because the sister was using it and the little child was sucked inside.
How the key would have helped in this situation?
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