A Tune to Make Them Follow (pdf, epub) by T.A. Lawrence

Piper’s job is simple.
Take the children.
Don’t get caught.
The rules she binds on herself; however… Well, they’re a tad more complicated.
Kidnapped as a child for her connection with the Old Magic—the one that grants her the ability to enchant others with her music—Piper knows good and well the rules of the Coup, the underground organization intent on bringing down the fae regime. As the Coup’s first “recruiter,” she’s supposed to take the strong, the good-boned, the clever, the healthy.