The House of Arden. by E. Nesbit

Edith Nesbit was a popular children's author of the late Victorian and early Edwardian eras in Britain. Though she was writing more than a century ago, her books nevertheless remain popular and are generally still in print. The House of Arden was published in 1908. Like her other, perhaps better known tales, such as Five Children and It, the story takes quite ordinary children of the time and plunges them into fantastical adventures.
In this book, two children, with the interesting Saxon names of Edred and Elfrida, aged 10 and 12 respectively, discover that due to the death of a distant relative, young Edred is now Lord of Arden. The estate consists of not much more than a little money, a crumbling castle, and an attached house. An old retainer tells them of a legend regarding the Lord of Arden and a buried treasure. Naturally they are eager to locate the treasure, which may help them restore the castle.
They discover a way to summon up the mascot of the House, a white mole or "mouldiwarp, " who enables them to travel back through time in search of the treasure.