Do You like book Megan's Island (1990)?
Megan Collier is used to moving around all the time with her single mother, Karen, and her little brother, Sandy. But this time is different. They are leaving in the middle of the night, and they have been forbidden to tell anyone where they are going. Megan had been planning on spending the summer break with her grandfather out on a lake, but when they show up there two weeks early, she is not the only one confused. Soon after arrival, her mother leaves, and people start showing up---people who are looking for them.
—Hailey