Most dangerous industrial processes like that I've seen go two ways: if a person makes it in the system emergency stops, and if it can't there's signage clearly

Most dangerous industrial processes like that I've seen go two ways: if a person makes it in the system emergency stops, and if it can't there's signage clearly stating brainless machines are at work (and always guards and such protecting actively moving parts). So in this case, the parent is likely right. The machine shouldn't be able to assume the field is empty. And since it's neither brainless nor can it have guards over the cutting implements, the next best thing is an area-of-denial zone around/in front of the tractor; pierce the zone and the tractor halts as fast as possible. And that pretty much leaves you with a vision system to watch for that zone violation. (It's easy to thoughtlessly make a minor mistake or forget something for just a second that otherwise appears to be a Darwin Award nomination. Constant vigilance is hard and impossible comprehensively, and while necessary, safety in layers is always a good idea. For example, I know at least some Kuka robotic arms with a cage denying access, yet robot will also notice a torque error and dump it's motors to a braking resistor, just in case the extra resistance was a squishy person - bypassing all interlocks is stupid and you'd have it coming, but death is non-OSHA-compliant.)