Hi, I went through this process a year and a half ago with my now 14 year old daughter.
Ended up purchasing a GSG1911 in .22 for her. That lasted a whole 2 range trips when I was informed that the .22, while fun, was also kind of boring.
When asked why - she informed me that it didn't have any kick and wasn't as challenging to shoot as my CZ in 9mm.
While the CZ (and just about every other 9mm) was too big for her hands (she couldn't drop the magazine without having to re-grip the gun). She ended up with a M&P in 9mm.
In the year and a half she has had the gun the back straps have been swapped out from small - to the medium one (she is almost into the large one).
Haven't used the GSG since the M&P arrived - she is most content firing away with the 9mm - and hitting just about everything she aims at.
Also, her and her friends seem to be attracted to the "cowboy" gun (my S&W 357 magnum). She has fired both .38 specials and .357 magnum rounds through it and is the second most frequent gun she shoots with.
L