Luke Lancelot and the Treasure of the Kings by Giles Andreae
Luke, Arthur and their sister Gwinnie are magically transported back to Camelot in the hope of finding their father, and returning Pendragon's Belt, with its powers of everlasting life, to its rightful place with the ancient Treasure of the Kings. The children set off on Luke's magnificent flying horse, Avalon, but they are followed by Dragor, one of evil Morganna's henchmen. They become trapped in a volcano where, to their horror, the children discover their father turned to stone and Morganna clutching Pendragon's Belt. But with Luke's bravery, Arthur's cunning and Gwinnie's quick-thinking, the children manage to trick Morganna, return the Belt and bring their father back to life - and home.