Like others have said, I`ve always seen it as a tradition or rite of passage, Kiara becoming an adult by proving her skill and strength in hunting. I`m sure she would have managed to bring down a prey given more time and training.
It just seems odd that the only real hunting seen in TLK (apart from Nala stalking Pumbaa) is solitary. I just think that it would mkae more sense if Kiara's rite of passage was with other lionesses.