The Wrenchies
"Sherwood and Orson should never have gone into that cave. That day, a door was opened from our world into a dark and profane realm...and earth's destiny was changed forever. In this demented future, whatever life remains on earth is oppressed by the evil shadowsmen. Only a gang of ruthless and powerful children called the Wrenchies can hope to stand against them. When Hollis, a lonely boy from our world, is magically given access to the future world of the Wrenchies, he finally finds a place he belongs. But it is not an easy world to live in, and Hollis's quest is bigger than he ever dreamed of."--From publisher's web site.