Bent arms on the kip will almost certainly improve with improved strength and getting the technique down - this comes through practice, conditioning, and drills. It comes slowly, but eventually it will be easy for her - I see many gymnasts learn a bent arm kip and slowly progress towards straight arms.
However, bent arms on the FHS vault and BHS are likely not to be an issue with power or strength, unless her run for vault is slow. Otherwise these are probably technique issues, but there are many different mistakes that can lead to bending the arms in either skill. For example, on vault: not getting the feet in front of her on the board, not understanding how to block through her shoulders, sticking the head out onto the table, reaching too low on the table, etc.
These are three complex and very different skills that take a long time to perfect, so I suspect the bent elbows are not caused by one general problem.