Not trying to add fuel to the fire but:
Certainly there have been many,many children who have had an uneventful, and even fun, ride on everything from a small riding mower to those huge ag machines. And if we /could/ look at the real numbers, the incidents are a small fraction of 1%. The issue revolves around acceptable risk. For ME, my risk factor allows for small child riding when no PTO, ground engaging etc. Once the tractor is in use we have a clear zone rule around the work area. Call me chicken, but after 30 years in the emergency medical business, I am unwilling to deal with the potential outcome.
In some ways, I would be more understanding of the " my grandpappy did it, my daddy did it, I'm gonna do it" if it were not for the 1-800-torts -r-us (appologies to those good atty's out there) suits where the equipment manufacturer gets sued for the op backing over/running over the child.
At the same time I have been teaching our child "stupid hurts". Been very careful not to allow him to get hurt too badly but, if he wants to drop a sledge on his foot by playing with it... We have a small place at 30 acres, 3 horses, a very large great dane (ok so I could say 4 horses) and a shop. Which adds up to countless ways for him to get an owie.
parental involvement is required.