Brain teaser math puzzle?

A camp counselor is hiking with Alice, Brad, and Candice. The four of them come upon a river that can only be traversed by riding a pulley chair that can hold either one or two people. Unfortunately, camp rules dictate that a counselor accompany a child on the chair. Furthermore, Brad and Candice are less than trustworthy, and the counselor does not want to leave sweet Alice alone with either of them. How do the four of them cross the river?