The Special Room by Gisela Woldenga

Ben and Kate are not too pleased to be left behind when their parents visit relatives—until a special room in their aunt and uncle’s house piques their curiosity. The room is haunted by a ghostly lady who needs their help. She begs them to go back to 1718 in England and retrieve a medallion that proves her innocent of killing her baby. She gives Ben and Kate her diary so they can read about her history and they decide to go. But how can they get there? And if they do, will they ever make it back?