Five Children and It

"Five Children and It" (first book in the Psammead Trilogy) is a classic of children's literature written by Edith Nesbit in 1902. 
Edith Nesbit was inspired by her own five children to write this enchanting novel skillfully told, and its warm and funny portrayal of a magical childhood has ensured its presence in print ever since.

Four children -- Robert, Anthea, Jane, and Cyril (the fifth is their infant brother known as Lamb) -- are playing at a sand pit when they stumble on a Psammead: an ancient sand fairy that will grant a wish a day, which lasts only as long as the sun is visible. Over the succeeding days, the kids learn how many ways that wishes can backfire on them -- they wish for money they can't get anyone to accept, beauty that makes them unrecognizable to their family and servants, wings that leave them stranded on a rooftop, a castle under very real siege by a knightly army -- and how to choose well and avoid having to lie.