The truth is that most small businesses pay under the table when they are just starting to bring on an extra person though no one actually admits it. I'm sure someone will jump in here and say they did everything exactly by the books from the very beginning, and maybe they did, but you can be sure they are a tiny minority.
It's not good long term strategy but sometimes it is good to do it til you figure out how to hire and figure out who is good because you CAN just say don't come back anymore and they are gone. No paperwork, not a bunch of w-2s to send out to people who made a whole $128 from you because they showed up for 2 days and then disappeared.
But only you can decide how you want to do it.
As far as being responsible for injuries, somehow I'm suspicious that a part time guy isn't going to be doing much beside weedeating and blowing and its not real likely he's going to chop anything off using those tools.