Flight to Trezarium by Trisha O'Keefe
Aya’s family has to move. The Triumvirate have picked up their sound and, even now, are sending the Borgs on patrol, sweeping the sewers for signs of anyone breathing the oxygen so vital to human survival. One by one, Aya has rescued a collection of human children from the Recycling Dumpster, as well as a mutant reject and an old robot named Blu. Raising them in the old sewers below Megacity on cabbage, water cress, fish, and frogs, she preserves the history of human kind by telling them stories about The Time Before when their kind ruled the earth.
But now, they must move like rats through the sewers under the city to the fabled place called Trezarium where they can see the sky and breathe the air. Aya remembers the way back from her parents who survived the purge, only to be recycled by the Triumvirate. Those tales kept her company when she was orphaned, warm through cold nights, and fed her when she was hungry. Now she would have this small band of wanderers do the same, salvaging the history of humans.