Where does she buy her clothing?
If you have Amazon Prime, they have an awesome selection of swimsuits. My DD has very narrow shoulders and is also tall and thin. This brand worked well for her in a 1 piece, so I'm not sure if the bottom would be small enough for your DD.
http://www.amazon.com/Gossip-Girl-Girls-Shock-Bikini/dp/B00AJNALUC/ref=sr_1_7?ie=UTF8&qid=1369185663&sr=8-7&keywords=gossip+girl+swimwear
Or try this brand:
http://www.amazon.com/Roxy-Kids-Girls-Strappy-Tankini/dp/B0098T3TUM/ref=sr_1_34?s=apparel&ie=UTF8&qid=1369186091&sr=1-34
I found awesome swimsuits at Dick's, Sports Authority, REI, etc. but they were all quite pricey. I'm not sure if there is anywhere besides Lands End that sells tops and bottoms separately, and theirs look more little girl to me.