Half Magic, by Edward Eager, 1954

Jane, Mark, Katherine, and Martha wish for adventures at the start of a hot, dull summer ruled by stern babysitter Miss Bick. They get more than twice what they wish for when a shiny nickel -- or is it? -- appears on the sidewalk. Wishes that are only granted by halves pose messy and inventive problems indeed, but the magic nickel has romance along with moralizing up its sleeve. A nostalgic and charming read, wonderfully witty; E. Nesbit's plots meet C.S. Lewis's genial voice. The four co-main characters are painted warmly and convincingly. Strongly recommended.