I like the idea of having a girl. Its a nice change up from other movies, and I feel like the personality that they gave Kiara would have only fit a female role. I don't think they would change the personality to fit a male role, and therefor changing the rest of the movie plot.
I still think it would have been nice to add a son to Simba and Nala, as that could have been played very well with the relationship between Simba the Kopa. But, I feel that the movie would have became too crowded with shorelines if that happened, so It was the right decision not to add Kopa in.