Remember, SWING WEIGHT is also very important. A lightweight racket with a high SW still swing sluggish for someone small of stature.
But I have to reiterate. The incoming balls she sees at 13-15 is nowhere near the speed of WTA balls. WTA pros, bigger and stronger than her, are using 11.5 oz rackets. Sharapova is 6'2". Serena is 5'9" and 175 lbs. Even Azerenka is 5'11" and 140 lbs.
And also, stiffness itself might be less important. A thick beamed racket of say...65 stiffness PLAYS stiffer and more harsh than a thinner beamed racket of 65 stiffness.
The modern game appears headed towards stiffer rated rackets....of thinner beams.