Very much an E. Nesbit book: family of children whose exploring discovers a somehow special place and leads to adventures, often of a magical nature. However, this book sits in an odd place of making all the adventures such that they COULD be natural causes of an unusually coincidental nature. In addition, one of the child characters is a sceptic about the magic, too. It's an interesting difference from her better-known books, but a less satisfying tack. It just doesn't hold together as well or compel the reader as much as some of her better books do.
Nevertheless, it was a pleasant read. In particular, I found it fascinating to see the treatment of lying by one of the characters. Unlike a modern children's book that might shake its head at such an act as bad but understandable from a child, here it is seen by the other children as breathtakingly dishonorable, and it results in some stringent consequences. Such is the Victorian regard for personal and familial honor. Sometimes these kinds of insights are worth reading even mediocre books of a bygone era.