True, but the kid is way more likely to get killed in a car, or drown in a bucket, or fall down stairs. The odds of a kid getting hurt riding on a mower (especially in a seat) are so slim it is not even worth discussing IMO. I have been almost killed many times, from being shot, hit by a car, and damn near drowned three times. Crap happens, and not enjoying time with a kid because there is a .00000001% chance he might get hurt IMO is crazy.
Statistically the most dangerous place for a child to be is in mummys car, it overtook the bathroom a few years back and before that it was around a swimming place ( pool/river/beach ).
But that is because most of them spend a couple of hours every day, in mummy car.
I do not advocate the cotton wool approach for either children nor adults.
I have always favoured the "let them get hurt & learn" approach rather that trying to make things impossibe to inflict an injury as the latter encourages a brain dead approach to your own safety.
As a male over 25 I have a fully developed brain so am capable of determining what level of risk I am willing to take, from guards missing on the compressor through to driving a mower without ear , eye or hand protection.
Children do not have the same amount of brain developement till puberty in a girl & around 25 in a male so it is our job to compensate for this till the child is capable of making their own decisions.
I drive mowers around 2 hour a day, testing repaired mowers before they go back to the customer.
Usually it is without any safety gear as I need to be able to hear & feel what is going on and it is amazing when clean up just how many little cuts & bruises I end up with.
A tiny cut on my old hard weathered hand is a deep cut on a child's soft skin.
Ad to that kids get excited, the bob up & down, look everywhere other than in front and wave their arms around , making it difficult for the driver to maintain proper control over the vehicle and that is before allowing for the tendency to watch the child and not where you are going.
The OP made his decision and while I do not agree with it he did go to reasonable lengths to protect his grandson physically but as mentioned earlier there is also metal considerations to the developement of the child's brain & how that child perseeves itself in the world