Traveling across space and time can take you to some dangerous places. Eleven-year-olds Jake and Ava are seasoned time travelers stuck babysitting their four-year-old cousin, DeeDee. After a day at the pool, DeeDee stoops to touch a strange coin in their driveway-and vanishes Oh, no What's there to do but touch the coin too? The twins find DeeDee in a narrow desert canyon-facing a huge, fierce...