I would get Tiagra. Even if she will grow out of it, if you really wanted to save money, you could just buy a bigger frame and move the parts, then wait till she's done growing to get her a Bike with Ultegra or even the Sram gruppos. If she really is into and wants to race, then a mid to high end carbon bike is not an option.

The thing about Tiagra that's great for an entry level cyclist is the Integrated Optical Gear Display. Sora has this too, but that's Sora.

You could, get a bike with Shimano Tiagra shifters, running Sora Front and Rear derailleurs.