You know, it sounds like the state has provided a workmens comp out for a guy with a mower trying to feed his family to me. Just following that exemption it sounds like a kid can mow a lawn for $25 and not be considered an employee. I guess so long as you don't give alot of requirements as to grass height, direction of mowing and such you can keep the independant contractor aspect alive and well.
When I was a kid I watched a man mow his yard and he hit something and his leg started bleeding. I saw the same man later on and he only had one leg. Now, that was a very long time ago, but you can get hurt, the liability question is what if something hit a passerby, could they hold you responsible, I'd think they could. In that case your liability coverage should kick in.
If the guy mowing got hurt the quest then is were you negligent? Does the yard have deep ruts, full of rocks maybe barried cables to trip over, if the lot is clear and you have not posed any danger to an ordinary capable person performing the job, I'd bet you would not be found to be negligent. That means it ain't your fault.
I'd say ask your insurance agent if your medical and liability cover temp employees on premises if they are not insured or required to be insured.
I have allowed teenagers to adults mow my grass for over 45 years, no problems yet! :)