Both my on-island girls would be totally readable. For Claudia, her biggest fear would be losing the people she loves like she lost her parents (and almost her brother). She's also rather leery of heights, underwater places, and dodgeballs.

Sam is . . . not afraid of much. She does have a reoccurring dream about a monster stealing her soup (though she might be over that), and she once dangled from a window-washer's platform 15 stories up with her best friend, so I'd say window-washer platforms (rather than heights, since she's also parachuted into Japan).