Its inexcusable, and ridiculous! No parent should endanger a child in this way under ANY circumstances. It is a blatant disregard for the machine, the child's life, and it is just plain stupidity wrapped in neglect and tied with a bow of ignorance and brazen disregard for life.
Every mower tells the owner/user to keep all people and pets far away from the machine, to not leave it running when stopped, to take the keys when not being attended by the owner/operator.
It's NO ACCIDENT when things like this happen. This is a case of preventable manslaughter caused by an ignorant, stupid, worthless, POS individual who deserves the maximum sentence allowed by law.
Suffer his brain with the consequences of his actions, for the rest of his miserable life. Too bad, play, you pay. In this case an innocent life was rendered meaningless by the idiot driving the mower. His choice- take responsibility for one's actions as a supposed adult.