Secrets of Ghost Island (Max & Me Mysteries #3) by Patricia H. Rushford

Max and Jessie get involved in a new adventure involving a trio of recently-orphaned children, who set up camp on nearby Ghost Island trying to escape deportation back to Mexico. In the meantime, a series of burglaries in town has police concerned that a gang of professional thieves is involved. When the girls get involved to help the orphans, they stumble upon the thieves' camp and are trapped.