I will admit that when we are at the park, etc. I am pretty much a supervisor.... I feel like I need to maintain a bit of a distance to see them as they all tend to spread out, lol!! Plus, they don't really seem to need me, and I am getting waaaaaay to preggers to be climbing and sliding.
Maybe I am a coward, but if I knew that she was working with an agency, I might go to them first. I would probably want to see if this was a pattern before doing that, though. Perhaps you caught her on a bad day, when she was a little distracted or whatever. Then again, as care providers, we can't really have those days. That is exactly when someone gets hurt.
If you feel like the kids aren't being watched properly, I would approach the agency responsible. Keep us updated!!