L.F.
I definitely think all parents should be supervising their children at play areas. And if their kids are unruly, I wouldn't be afraid to tell them to knock it off if their parents won't. If there are park employees around, I would notify them, too.
However, it's not fair to expect that just because a child brings his favorite toy to a play area, it is fair game for all the kids there. It is a public place and people can bring what they want. It's up to other parents to teach their kids that they can't always have other people's things. Being a kid doesn't make you entitled to have what everyone else has. That's a lesson best learned early.
.....LF