Sad story, and I feel for the little girl, but...
I learned to drive a mower (and pretty much everything else) BECAUSE my Gandpa had me on with him.
Mowers, tractors, dozers, boats, motorcycles, jeep, firearms & chainsaws... all were taught to me by Grandpa before the age of 10... any one of them could have taken me out with a bit of bad luck and/or by my own stupidity.
It was just a bad day for them...
Enjoying life involves risk.