Disciplining your kids?

So I've been taking care of my 2 young nephews everyday for the better part of a year now. I make sure they get outside a lot, read with them, etc.. but when it comes to disciplining, I'm not sure. I try to teach them right from wrong, but if they refuse to listen, is it alright to hit them? (just like a spank, or a smack on the leg or wrist.) I've seen how some kids are raised up to be, and it's sickening. Nowadays, things like "child abuse" worries me. What do you think?