I would just say "the library toys are for everyone. DD is having a turn right now, and when she's finished, you can have a turn." If he'd taken it, I would take it back, pretty much repeat what I'd said, maybe along with something like "it's not okay to snatch toys away" and then distract one of them or the other. My DD has a tendency to just shrink in this kind of situation, and I won't let her be bullied. If another kid isn't really being supervised, then I often assume that role. If the other parent doesn't like what I say/do, well, they can step up.