It's a tough call. I am having the same dilemma, but with DS1 who is nearly 5. There is one particular child at the centre who is constantly causing trouble and "bullying" the other children. However, with most centres not willing, or able to exclude said children I'm not sure there is really much to be done. At ours they talk to the kids, and the parents if there are major altercations, but then it's down to the parents to speak to their kids at home. Now us being responsible parents, we explain what we deem acceptable and unacceptable behaviour, but who knows what, if anything the other child's parents are saying to them. DS1 can usually tell me who he's had the issue with (the centre never does, just say that he and "another child") and even if I knew the other child's parents well, not sure I'd feel that confident to approach them about it.....especially if I get the impression that it's not that big a deal to them!!!
Yes, kids will be kids, and they will not always play happily together, but if it is consistent or it's the same child over and over then I'd be looking to speak to the Head of the centre to arrange a meeting and discuss your concerns. We did this when we learned that DS1, along with about 4 other boys, were not being very respectful to centre property and basically just being "boys", but as his parents we felt that it needed discussing because he knows better and we wanted to see if there was anything more we could do, or that they could do to "keep him in line". They came up with some new ways to focus their attention and within a week the behavior had stopped.
So it's a tough call, but you could maybe sound them out by mentioning it to them one day and seeing what kind of a response you get. Then depending on that, determine where to go from there; it's hard, leaving them with other people, but at the same time it's great for them to be socialised and learn how to deal with that stuff themselves. Daycare has done a world of good for DS1, but he has picked up some habits I had rather he didn't have. LOL He's off to school shortly so I assume it's only going to get worse from here on in. At the end of the day, it probably won't be the worse thing that happens to her in life (DS1 broke his arm when he was 18 months old)so a few scratches, in the giant scheme of things will go largely unremembered. If you have any concerns for her safety though, I'd do something immediately about that, but maybe take a wait and see approach for now?