The Duchess of York has signed a contract to write two children's books, scheduled for publication at about the time of her own child's first birthday, Simon & Schuster announced Monday. The 28-year-old duchess, the former Sarah Ferguson, is married to Prince Andrew. The couple is expecting their first child in August. The two children's stories, about a helicopter named Budgie, will be aimed at very young children and will be illustrated by one of Britain's top children's book illustrators, according to the publisher's announcement. The American company is planning to publish the books in August 1989 in association with Simon & Schuster Ltd. of London. The duchess has been working on the books with an editor in London for the past few months, according to the publisher. A portion of the proceeds will be donated to charity.