I agree that Kiara ordering Kion away from the tree she wanted to use to sharpen her claws on was in poor taste, but as you said They are both quite young, and that is the kind of things siblings do.
[quote="Amadi_the_Guard"]I'm sure Simba could have found someone else to take care of the kingdom (some adult lioness from the pride, relative...), and waited a while longer before he allowed Kiara to practice ruling.[/quote]
I agree, but perhaps Simba thought that ruling during such a short trip would be good chance to give Kiara a taste of her future responsibility.Without overwhelming her. Another thing to consider is that, by Lion (and more importantly in this case Lion Guard Standards) Kiara is perhaps considered older than we are perceiving her to be. After all (and I
hate to use this as an example) if Kion is old enough for Simba to have given him
The Talk,

Perhaps a Lioness does not have to be as old as we think she should be to rule a kingdom.