Kids are curious. If the padlock has been broken off, and the kid is strong enough to lift the cover/door, this is a real problem. I'm surprised it hasn't happened more often. Really young kids will wonder why it hums, not knowing that means danger.
I have to assume the lock was broken in this case. Or maybe it just didn't get locked properly one day (I've seen that happen with padlocks). So who has the responsibility to check these on a regular basis? It might be hard to point blame somewhere.
Maybe they need to be made with a locking mechanism that is harder to break. The smaller single phase pad transformers I've seen are locked with a padlock. Maybe some kind of recessed key lock system is needed, instead.
