Curse of the Ruins — Gary Paulsen

“A few hundred yards ahead there is a long wooden bridge that the donkeys will be afraid to cross. But do not worry. After you cross the bridge, the ruins will not be far away.”
 
Cousins Kate, Sam, and Shala quickly see why the donkeys are so afraid to cross the bridge -- it's old and rickety and spans a deep rocky canyon. But they have to get across it. They have to find Katie and Sam's dad, an anthropologist who's studying the ancient cliff dwellers at El Debajo. Dr. Crockett was supposed to meet the cousins at the airport in San Marcos, New Mexico, but he never showed up. They don't know whether he has enemies who might have kidnapped him or whether there really is a curse on the ruins of El Debajo. But they're about to find out.