I also think that Simba is overprotective towards Kiara. But, if we consider the danger that Zira provides for the pride, especially for Simba's own family, would'nt you also want to have your only child watched by your closest friends so that no one would harm her?
I don't know if Timon and Pumbaa would have been able to stop a fury outlander-lioness like Zira (moreover, I REALLY doubt that) but maybe they could have helped her escape in an emergency situation as Pumbaa is gifted with his very special....super-weapon, if you know what I mean.
