Five Children and It (Puffin Audiobooks)

Robert, Cyril, Anthea, Jane, and Baby brother "Lamb" find an ancient Psammead sandfairy who grants them one wish per day. But whatever they wish turns wrong. They try "beautiful as the day", "wealth beyond avarice", angel wings to fly, defending a besieged castle, raiding Red Indians, and more. Diamonds for Mother on her return is the last straw.