A girl has a secret refuge, either in the attic of her house or a loft in a barn, where a wise, nurturing grandmother type lives. There is a beautiful, soft bed where she can rest and a magic tub, where she can wash away all cares and fatigue. But when she shares the refuge with another girl, this girl tells her there’s no grandmother, the bed is just a pile of straw and the tub is just an old wooden bucket. However, the main character still believes what she has seen and experienced. I think the book is British and was probably written in the 30s or 40s, although I read it in the early 50s. It may be part of a series. I remember it was included in a New Yorker Christmas books for kids article in the late 80s (should have saved that article!) so maybe some kind of anniversary edition was issued then.