I can see how it could happen without the MIL being negligent, actually.
She could well have been holding him by the work top, and he grabbed the cable from the electric foot plate, pulling the base from underneath the kettle and upending it.
I am paranoid about burn injuries, because I did a placement on Salisbury Burns Unit when I was a student nurse.
Still, I can think of many incidents that were 'lucky escapes':
-Having coffee with a friend in a coffee shop. Friend's baby DS lunged forwards and grabbed the saucer. Fortunately, it tipped the fresh coffee towards ME and the friend instinctively pushed herself away from the table to distance them. (We were all ok...I did the same).
-A friend had a cup of tea on the side, then answered the phone. Her 4 year old tried to reach something and tipped the tea all over her. She still doesn't know how she didn't get burned.
-Another friend was starting to prepare to cook while her DD watched. She cut some onions, then reached over for some oil to put in the pan. Less than 5 seconds. Out of the corner of my eye, I saw a glint of brightness. I realised that her DD had reached over and picked up a big knife by the blade. I grabbed her wrist and squeezed, hard, to make her release her grip. Luckily it worked.
-DD2 heard a sudden noise and ran away from it, jumping onto her sister's bed, which was next to a radiator. The radiator had been removed because we were decorating - her back was sliced by the radiator bracket. She still has a scar.
-I managed to smash DD3's head against the corner of a wall on Boxing Day, playing a silly game. We had to get it glued in A&E.
-DD1 was playing at school and broke her hand.
-DD1 was playing at respite, 4 days after her hand cast came off, and broke a growth plate in her ankle.
-When DD1 was a baby, she tried to eat a piece of mushroom and choked.
These were all incidents that could have been so, so much worse. We were lucky.
We're all careful people who make mistakes and regret our momentary inattention.
In case anyone doesn't realise, the above incidents are based on accidents involving 6 children over a 10 year period!