The King’s Riddle (Land of Miu, #2, 2nd edition) by Karen Lee Field
Princess Alara and her guard-in-training, Siptah, have done something unthinkable. By accident, they activated the volcano deep below the tunnels in which they live. If they can’t stop the volcano from erupting the whole of Miu will be lost.
But that isn’t the only problem they have, their fathers are missing and time is running out, so it’s up to them to fix things by themselves. A clue found in the king’s chamber suggests their human friends, Kate and Emma, must come back to Manu to help them. Can the group of friends overcome their fears and frustrations and solve an ancient riddle in time to save the Land of Miu?