Her lyrics in "My Lullaby" refer to Simba's death only. The only thing we hear about Kiara is "Simba's daughter squealing in my grasp". That is interesting as, if you recall in Lion King, Scar's goal was to take out Mufasa AND Simba to "legitimize" his reign. Unless Zira was planning to take out both Kiara and Kion (who wasn't even cannon when LK2 was written!) , it would seem that her ambush of Simba, HAD it actually worked, would only have succeeded in making Kiara the new monarch (unless Nala would take it as Kiara might not have been of age, not sure how that would have worked.)
However, assuming the throne would go to Kiara, that would make an interesting point of "squealing in my grasp" may have indicated a scheme to make Kiara forcibly be the wife of Kovu (Perhaps Zira had been afraid that Kovu might not agree to directly kill Kiara (they were seeming to become friends before Zira and Simba turned up and ruined everything) and may have even gotten Kovu, who may have had an idea of marrying Kiara, to further go along with the plan, telling him that as long as Simba was alive, he'd never let their marriage happen, even if Kiara and Kovu wanted it. Interestingly, the definitive point that Kovu turned on Zira and her plan was when Simba seemed more open to him and was willing to invite him inside the den, indicating he was getting more open to him being with Kiara.)
Of course, Zira may have always planned a brute force assault to wipe out anyone loyal to Simba and just take it the brute force way as her lines "the battle may be bloody, but that kind of works for me", but she may have been referring to kidnapping Kiara as Simba has her crazily guarded. Zira just had the good luck of picking the one time (hunting, and not even on her own, as Timon and Pumbaa were tasked to follow her) that Kiara was most vulnerable and out of Simba's guard to stage the "fire rescue". Getting through to take Kiara prisoner would have certainly been messy under most circumstances! (Who knows, perhaps, as Zira didn't mention killing Nala either, her plan was to kidnap the both of them and threaten to kill Nala unless Kiara agreed to marry Kovu, which she'd probably have done to save her mother's life.)
Interestingly, after Nuka's death and Kovu's betrayal, it seemed she no longer had a male to put on the throne, and that may have been when she decided for a direct brute force attack on the Pridelands, as, at that point, she really had no way to force her way to the throne through marriage or other means.
Interestingly, when Vitani attacks Nala, she says "Where's your pretty daughter Nala?", which may mean that by that point, Zira had tasked Vitani, her chief lieutenant, to kill both Nala and Kiara, thus eliminating the immediate successors to the throne once Zira had taken out Simba.
Interestingly, that would make all the more sense as to why Vitani refused to fight: she felt guilty as she had planned to kill Kiara yet Kiara was now telling her father and his side NOT to tell them but to let them in instead. (As far as we know, Kiara and Vitani had never personally met until that point and Zira may well have told her all sorts of lies as to how evil Simba's daughter was.) Well, that plus the death of Nuka hit home as to Kiara's words that the fighting was pointless.