I suggest that you give them concrete rules that they can adhere to. Saying things like, "Be careful, don't break stuff, don't lose things" I think doesn't register. It's too vague. You could look at a situation and the two of you could have an argument on whether or not they are actually being careful.
Saying things like, "never set a sling on the ground, cover glass doors before you stumpgrind near them, or don't fill up saws in front of the truck" are concrete statements that you can evaluate. There is no middle ground to these types of rules and anyone can tell if they are following them or not.
love
nick