Aggressive or Assertive?

Hi all, as some of you know I've got a couple of foster ferals. I've had them for a couple of months now as they had health issues and were slow to socialize. They're getting bored of being in the bathroom and try to explore the rest of the apartment, so they get to go out from time to time.

The problem is that Nova doesn't seem to like them being in her space. She doesn't seem to have a problem with them when they're in the bathroom, and she's even somewhat curious about them. The kittens are submissive, rolling around to expose their tummies to her. However, this doesn't satisfy Miss Nova as she tries to bite them in the belly and in the neck, and she swats at them too.

I intervene because she's twice their size and could seriously hurt them by biting their vulnerable parts. Am I misinterpreting her actions? Should I not intervene?

The new kittens I bring into the house I don't intervene after the initial introductions. They usually work it out themselves. If one gets too much the new one will just run away. Good luck.