As purveyors of inexpensive food, the Childs Co. has had as many ups & downs as its flapjacks. On the other hand, New York's Louis Sherry, Inc., makers of high-quality candy and ice cream, has had an income as substantial as most of its customers. Last week) 61-year-old Childs decided to upgrade its food and its balance sheet. For "more than $2,000,000" it bought upwards of 90% of the stock in Sherry's from Mrs. Lucius M. Boomer, widow of the Waldorf-Astoria Hotel's longtime board chairman. (Boomer founded the confectionery company with the late Louis Sherry after Sherry's fashionable Fifth Avenue...