Just get the plain Coronation Ace blade, the K-pick option widens the toepick area, to make toe jumps (allegedly) more secure. Good technique does the same thing.
If you want to be able to move the blade to the next-new boot, measure the boot end to end and that's the blade size.
You can't make it any longer or it would be dangerous.
There's no guarantee that the new blade will fit her next-new pair of boots, you understand. It just takes one wild growth spurt or a change in boot model to put those plans aside.