The children were playing with it unsupervised - presumably this is why there was a camera trained on them, but no one was actually watching.
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