Shadow Jumper: The Quest by Cait Shea
Sucked into a time warp, Sinead, Kailum, Yolanda and Elicia, four cousins living very different lives around the world, are thrown together and embark on an incredible adventure, uncovering a mysterious future where children do not exist. The cousins become fugitives as they elude hybrid beasts, angry mobs and a tyrant ruler and learn that they must work together if they are ever to find their way back home.