Anvil of Stars — Greg Bear

The Ship of the Law was made of the fragments of Earth's corpse, a world in itself, cruising massively close to the speed of light, hundreds of years from the dust and rubble of home...

And aboard were 82 mortal exiles sworn to find and punish the Killers who murdered our world.

They were more than children sent on a time-bent crusade; they were avenging angels, soldiers trained but not yet tested, given access to incredible powers they did not completely understand. Eventually they would use that power if they were to fulfill their mission, and some of them would not survive.