I read it as a child so it must have been written by Nina Bawden, Nina Beachcroft, Joan Aitken or a writer of that era/genre.
It was about three children who for reasons I cannot remember (orphaned?) are sent from their home in Africa to live with a distant relative in a seaside B&B in England. I vaguely remember a jade horse called Pin appearing somewhere...
Warned you it was a long shot...