If she's going to grow out of the bike, I wouldn't get her anything too nice. Sora is a perfectly fine groupset if well tuned. I'm sure the newer Sora, which is nine speeds, is even nicer.

Group compatibility depends on the shifters. If you have a ten speed shifter, you'll need a ten speed cassette and chain. All the derailleurs should work fine... if you wanted to use a Sora derailleur with DA shifters you could, or DA derailleur with Sora shifters. If you have a triple shifter you can use it for both doubles and triples, however you can only use a double shifter for a double crankset.