This is an interesting point. She mentions "squealing in my grasp" but doesn't further elaborate.
If Zira intended to kill Kiara, though, I think that Kovu would have told her when he fell in love with her. I don't think Kovu fully cared for Simba until Simba had told him the truth about Scar and all of that. hence why Kovu was content NOT to reveal the plan ahead of time to even Kiara (if she loved him, I don't think she'd outright tell her father, as she knew Simba would banish him.)
One possibility is that Zira intended to force Kiara to marry Kovu after Simba's death, thus making Kovu the king of Pride Rock by marriage. If so, that might be why Kovu wasn't too concerned about Kiara being in danger.
The only time that Kiara was in direct danger from Zira, unless Zira tried something when Kovu and Kiara first met as cubs (it's been some time since I've seen Simba's Pride), was at the final battle when Kiara jumped at Zira to stop her from killing Simba.