American, set and published in the 50s, I think.
There's a family: mum, dad and three (?) children. Their nanny/housekeeper's recipe for bread pudding does very well in a baking competition, and she goes to New York for the final. For some reason the parents have to go somewhere else at the same time, so they get a live-in sitter for the children.
The children do not take to the sitter. She means well enough, but makes them eat Health Foods etc. The youngest child has a magic kit. He makes an effigy of the sitter and puts it in a model bed. She promptly takes to her bed, though she is not ill, leaving the children to look after themselves.
Then, of course, they have lots of local adventures, some (most? all?) of which involve the magic. The only one I remember is when they make it snow, but only within the town boundaries.
Anyone recognise this? I read it in England in the late 70s/early 80s, and it was an old, secondhand, hardback book.