Trial by Dragons: The Warlock's Child Book Four

Two of Australia's successful fantasy writers, Paul Collins and Sean McMullen, collaborate on this action-packed fantasy series.
It's hard to save the world when you are in jail. Dantar and Velza, children of the warlock Calbaras, need to escape from jails six hundred miles apart to stop their father unleashing a catastrophically dangerous spell. Both will soon learn that they are not just special, they are also very dangerous.