I suggested the circuit because :
1) its high quality , but much lighter than many others
2) the adjustable shoulder strap panel gives it a torso fit range from about 13-18"
3) the big side pockets and mesh back pocket are just as great for kids usage as adults
4) the resale will be very good if you decide to get rid of it, so in the long run cost is probably a wash. You will eat the full cost of many cheaper packs.
5) she likely wont carry >20 lbs, and need any more pack/padding than that anyhow.
6) can get it with S curve straps, or change them in future as she gets more womanly
7) the service is pretty much unbeatable. If you want modifications, changes, repairs, customization, etc it can be done.