Children of the Lamp, #4: The Day of the Djinn Warriors book by Philip Kerr

Djinn twins John and Philippa are off on another whirlwind adventure that takes them around the globe and into unknown worlds. And it's a race against time as they attempt to rescue their mother from her fate as the Blue Djinn of Babylon. An aging curse has been placed on their father and if the twins are gone too long, he'll rapidly become an old man. Meanwhile, museums all over the world are reporting robberies of valuable jade from their collections, as well as bizarre hauntings. As the twins and their friends travel around the globe on their rescue mission, they notice that something very strange is happening: An evil force has awakened the terracotta warriors created by an ancient Chinese emperor, and someone with very bad intentions has cast a spell possessing the soldiers with wicked spirits. And now, the very fate of the world hangs in the balance. Can the twins stop the mysterious terracotta warriors, rescue their parents, AND save the world before it's too late? Published By Orchard Books (NY) on 2008