The Spurs (Golden Armour, #3) by Richard Brown

Cassie, Keiron and their father are now on fog-bound East Island, a magical island where flying moonhorses and horrifying moonghosts delight and terrify the people in the shrouded city. Their search for the Golden Armour's Spurs is not going to be easy. Forced to seperate from their father, the twins are sent to an isolated school where the staff are stern and the rules frighteningly strict. Forbidden contact with the outside world, they feel imprisoned. But there they find a secret band of children and a legend in which they are to play a crucial part. Together, they pit themselves against enemies old and new in their fiercest fight yet, to secure the Spurs and save the island.