I've had a exacto knife in my hand since age 5. My dad told me, if you mess around with this, you'll get hurt, so don't mess around with it. He taught how and when to use it and also what to do if I did happen to cut myself. I've sliced open my forefinger twice, thumb 5 or 6 times and the webbing in between my thumb and forefinger, luckily I missed the tendons and stuff, somehow. Recently, I've dropped a dentist tool and it went on top of my foot, through my jeans and sock. That really, really hurts! I would not recommend. lol.
Just tell your kid, hey, if you mess around with this stuff, you're going to get hurt. And if you do get hurt, do yadda yadda and yadda yadda. Just let them know, this can hurt you if you don't respect it. And if they can't handle it, give them the sprue cutters and a file. And then try again later.