I find it to be a little farfetched if you ask me. It wouldn't surprise me that a lioness would attempt kill a hyena. since lions and hyenas are natural enemies like cats and dogs. Although in nature documentaries we usually see the male lions attacking the hyenas, and the lionesses wouldn't usually bother with hyenas unless they're defending their cubs or stealing a kill literally from under the hyenas' snout. Fun fact: Lions are mostly scavengers they usually hunt when needed because they don't have the stamina or speed to properly hunt. And I don't understand why she would anyway eat Banzai's parents anyway for a few reasons.
1: She lives in a well fed pride- Since Sarafina lives in the Pride Lands there are essentially a variety of prey that she could hunt for with her pride sisters. And with all the choices of prey she has such as a gazelle, antelope, or wildebeest why would she waste her energy on a couple of hyenas that could probably only feed Mufasa and Sarabi.
2: She couldn't risk it- Just like lions, hyenas live in a group called a pack. So if she tried to attempt to eat Banzai's parents let alone lays a paw on them then it's guaranteed that the rest of the pack would use that as an excuse to rip Sarafina apart and then she herself becomes the meal. And even if the pack wasn't there to back them up the hyena couple could do some serious damage on their own. One set of hyena jaws can easily break a rhinoceros leg with one chomp if given the opportunity. So, imagine what two sets of jaws could do to her.
3: The hyenas themselves couldn't risk it- Since it's obvious that Mufasa is willing to protect his pride members (Remember the Elephant Graveyard scene when Mufasa practically attacked Shenzi, Banzai, and Ed when they came near Simba and Nala.
) Then it is without a doubt that Mufasa will come to Sarafina's rescue. If Mufasa can pin three hyenas down on his own then pinning two down without the rest of the pack should be a cinch.