Scrub Kids, Book 1, Molding Destiny

A group of kids go wandering around, looking for something to do. When the group realizes that one of them has gone missing, they all come together, working tirelessly trying to bring him back. In the missing boys place, the group finds a mysterious object, discovering it as a gateway between space and time. Meeting all kinds of friends and foes along the way, finding themselves on new worlds they'd never imagined, Zac, John and James set themselves off to find their friend AJ and bring him back home.