Midnight City by J. Barton Mitchell

An alien race known as the Assembly has taken over Earth, bringing the Tone with them. The Tone is a telepathically-broadcasted signal that takes over the minds of everyone above the age of eighteen. This means there is one group left untouched: children. Holt Hawkins, a bounty hunter, is one of the lucky few: he is immune to the Tone. His current target, Mira Toombs, is playing hard-to-get. Many bounty hunters before him have attempted to capture the infamous Mira Toombs, yet none have succeeded. But, when Holt finally meets Mira, neither of them can ignore the instant connection they have towards each other. Not long after Holt takes Mira as his prisoner, an Assembly ship crashes nearby. Inside, Holt finds a little girl who remembers nothing except her name: Zoey. As the three of them make the treacherous journey to Midnight City, they encounter rogue alien armies, pirates, freedom fighters, and the amazing powers Zoey is beginning to exhibit. As time passes, Mira and Holt begin to realize that Zoey may just be the key to stopping the Assembly once and for all.