Does she understand what hurting is? My DS knows what "ouchie" is now and that helped us a lot. He is very aggressive with me (all his attacks centered around me but he was a little rough w/other kids too). I would grab him and say "look at me"...he hates this bc he knew it meant he was in trouble...and say, "you gave mommy an ouchie. We don't bite/pinch/pull hair bc it's not nice. Be gentle!" Then it was TO for whatever it was. I think he hated the little speech more than the TO tho!
Also, DS loves the play gym at the rec centre. Whenever he was too aggressive there, it meant we left IMMEDIATELY. He has been very good there now.
He still likes to jump on me or hits me when he is upset but there has been no biting/hair pulling/pinching for a few months now :)
Also,I would warn the parent that this behaviour has to be resolved if she wants to keep you as a babysitter. It doesn't sound like she is taking it seriously!