I read the book in the late 1970’s early 1980’s. It would have been a fiction/science fiction paperback book from the local library (Manchester, Georgia). I remember a female protagonist child (and her brother) who were lost members of technologically advanced race studying the planet but were members of private tribe? She was being called back (dreams). She had to run away from her tribe. While in route her brother stepped on something in the sea (perhaps a sea urchin) (Spoiler alert! ………. and died.)