There are two approaches to this task. One is to buy a travel-friendly pack that has railway stations, jetliners, and hostels in mind. One of my favorites: Lowe Alpine's Voyageur 75 APS ($249). It's a travel pack based on a very good backpack, which is a good thing as it ensures the suspension is up to snuff. The Voyageur is fairly roomy (4,600 cubic inches), its suspension zips out of the way when it's traveling as checked luggage, and several handy pockets and compartments provide good places for storing passports and other necessities. Unfortunately, Lowe Alpine is discontinuing the Voyageur, though a number of retailers still have some in stock. Another travel pack that's skewed toward the "travel" side of things is Eagle Creek's Switchback Plus ($249 in size large). It converts from a wheeled suitcase to a backpack, a switcheroo that can be pretty handy. Eagle Creek stuff is uniformly well made and well designed, and the only drawbacks to the Switchback are its smallish size (4,200 cubic inches) and the fact that it's not as functional a backpack as the Voyageur.
The other avenue is to buy a backpack and do one or two things to make it more airline-ready. A superb all-around pack is Gregory's Palisade ($290), which has an excellent suspension and is extremely ruggedly built. A top pocket and sleeping bag compartment aid organization, while plenty of tie-down loops will give your daughter more places to stick stuff. Plus, it comes in several sizes, unlike the one-size-fits-not-everyone approach of both aforementioned travel packs. A size small should work for your daughter, and even at that the Gregory is slightly larger than the Voyageur. The only problem is that your daughter will need to figure out how to get the straps out of the way, as most airlines don't like that sort of thing. Solution: Buy a light duffel back that the pack can fit into. At the airport, the pack goes in the duffel, duffel goes on board, and everyone is happy.