As I posted on the thread "Rumors of the Cub at the end of TLK":
When SP first came out, I was under the impression that the cub was just Kiara. Sure the books said that Simba had a son, but I just chalked it up to "well, they didn't expect a sequel when the books came out, so it's just a minor error.
Then, once I got TLK6NA box set, there was Kopa. And I kind of started to think that maybe the cub WAS Kopa. Certainly, Simba and Nala are older when Kiara is presented (as compared to the presented cub at the end of TLK). Also, Kiara is by FAR an older cub than "Fluffy" is. (Watch when she swipes at the butterfly or whatever - her expression is very mature for a cub). And little hints in the dialogue made me think that the intial cub was Kopa and that he was murdered.
Zazu talks about "murderous" outsiders. So who did they kill? Sarabi maybe but it could lean towards a murdered Prince too. Simba talks about how you "can't turn your back" on the outsiders. Again, it makes me think that maybe the Outlanders were responsible for his first cub's death. When Kiara asks "why?" Simba says "never mind." Kiara is too young at the time to know about the brother she never knew and it is too painful for Simba to get into anyway. Simba is also wildly overprotective of Kiara. Fatherly instinct? Maybe. Or does he fear losing another cub. When Zira and Simba first meet when Kovu and Kiara are playing, she pushes her son to him offering "if you need your pound of flesh." Why would Simba demand such a high price of those caught in the Pridelands? Why not just chase them off when they break their exile? Is he THAT harsh of a ruler or does he have a good reason to demand such a repayment? Sure, you can argue that he fears exactly what Zira has in mind (that is a hostile and deadly overthrow). And I am sure that such thoughts ARE a part of Simba's actions. But it seems to me that that cannot possibly be the entire reason why Simba acts/says what he does. There must be another reason.
And so, I am now of the mindset that the cub at the end of TLK is, in fact, Kopa. I do not for one second believe that it is Kiara.
I also agree that Kiara was written as a female in SP to a) make SP a different movie (father-daugther as opposed to another father-son romp) and to b) appeal more to female viewers (especially considering how love-heavy the movie is).