I think playfelt offered great advice. Let the parent know that you'll give this a few more weeks to see if this little girl can adapt to your home and follow your rules, but that the parent needs to be firm at home as well. But if it continues, you would have to terminate.

I had a little one that was 2 who was a biter. I explained to the parents, that I would give it two weeks and that they needed to address this at home firmly too. Timeouts are difficult at this age, so what I would do is make them sit next to me. If she had a toy at the time, I would not let her have it. Talk firmly. I also spent those two weeks each and everyday with them at story time, we would sit side by side and talk about being gentle. Then we would take our hand and lightly touch the one sitting next to us, quietly saying gentle. Every now and then, say to the girl when she is near the others, gentle...as a reminder.

Good luck.