Rainstorm by Barbara Lehman

From chapters.ca:
It can be lonely sometimes on a rainy day in a big house with no one else around and there's only the quiet to keep you company. But if you find a key, a mysterious key, that leads you to an unexpected place... chances are your afternoon is about to get a lot more interesting. Caldecott Honor winner Barbara Lehman recalls many stormy days in childhood when she was completely content just to play or read indoors, feeling cozily sheltered inside during bad weather. This book, however, was inspired by her memories of the other kind of rainy days, the ones of feeling utterly restless and trapped and just wanting something to happen...