Yup, still in agreeance with playfelt. And, yes, if I was around any kids I care for even outside of daycare hours, I would probably still act exactly as I do normally. But admittedly, since I started my dayhome, I will parent anyone's kids. (I have been known to tell kids at the grocery store that they shouldn't put something in their mouth, etc. LoL)
I don't know if I would ignore the bad behaviour, I have zero tolerance towards aggression, etc. I don't think ignoring works with hitting, fighting, being rough. But a non-reaction seems like something that will work, eventually, as he learns that he can do that all he wants and the same consequences will occur. And he won't like it!
If you really feel like the other kids safety is at risk, then that has to come first. If between you and the parents, the behaviour does not resolve, then it may not be the right place for him. There may be something causing this that you cannot fix?