The Moonstones by Jean Thesman

Jane visits Royal Bay, a small town along the Pacific Ocean with her mother in order to clean and repair her deceased grandmother's house. Her relatives, Ricki and Aunt Norma, are supposed to help but instead make everything harder for Jane and her mother to bear. But at night, Jane and Ricki sneak to the small amusement park along the beach and meet with a few of the natives. There, Jane meets Carey and everything in Jane's life changes.