This is something I worry about every time I get on my tractor. Most of my work happens in the back yard beyond the current lawn area, so my 3 year old is not allowed to step off the lawn while I'm on the tractor, nor is she allowed to come talk to me while I'm working (dirt work). But I still always keep an eye out to see where she is in the yard when outside - I've even stopped and climbed off the tractor a number of times to go track her down, and found out she went inside. And I constantly remind my wife and the older kids to keep her in their sight while I'm working on the tractor.
She's also not allowed outside at all while I'm mowing - not only can a child be run over, but they won't fare too well if hit by something thrown by the mower. That's why the mfr's make the chutes so that the clippings are sent out in a downward arc, and why I would never flip the chute up while mowing, as some folks do.