Helen Cresswell was an English writer of over 100 books for young readers, perhaps best know for her series about the Bagthorpe family. She also wrote a few timeslip books, that are rather magical in an understated way, and which are heavily atmospheric and rather haunting, though not my personal favorites.
In Up the Pier (1971), Carrie and her mother are staying in a Welsh seaside resort town, but it is off-season, and chilly, and wet, and lonely. Her father is off for work reasons, and the family is between permeant homes, so everything feels off kilter to Carrie. But then on the pier (the British sort of seaside pier that's kind of like a boardwalk but sticking out into the water rather than alongside it, with boardwalky sorts of amusements and games on it, but in this case, shut down because summer is over) she strikes up a friendship with a boy whose life is even more off kilter. He and his parents and grandfather have slipped forward from the early twentieth century past...and his grandfather, an magician of sorts, things that he must have worked the magic that did it. But he can't figure out how to send them back.
And the ticket taker of the pier, a grumpy old man, who's from an old pier family himself, and has no family, is rather too glad to have the company trapped there on his pier...
The family struggles to make a home for themselves in one of the empty buildings, and Carrie helps with shopping, and enjoys the company herself, but at last it becomes to clear to her why they came to be in her time, and how to send them back again.
Yes its magical, but its somewhat dampened by the fact that the atmosphere is a chilly deserted waterfront; every day seems gray. Still memorable and fascinating; the magic is real, and the setting feels awfully real too. If you are a time travel fan, it's one to look for. The ways Carrie helps the family adjust (to a point) was my favorite part. Like other time slip books by Cresswell, I felt it was too brief (it's only 144 pages) and the main tension of the story was external to Carrie, so not quite as emotionally absorbing as I would have liked. She herself is never at a pivot point of suspense, except when it comes to sending the family back, so it ended up being a bit of a shrug read for me.