Were it my child, I'd explain what the tool was for, how to use it properly, make sure the kid understood the consequences of misuse, and watch them the first few times they used it to make sure they followed the appropriate safety practices. After that, I'd trust the child to know how to use the tool and why the safety gear was important.
I would always supervise kids with power tools just as I would with kids with a gun. I'll bet that your kids like most would be sensible once they had been taught but accidents do happen. Sure kids need freedom to learn, but they can get that by you both working together on a project with you taking a back seat.