Beauty and the Beast, which opened the Muny season, was an ebullient evening of tuneful family fun. True, too many Muny productions seem antiquated and lumbering. But if any lesson has trickled down through the institution's 86-year history, it's the need to reach out to youngsters. The Muny staged its first children's show (Victor Herbert's Babes in Toyland) in its second year (1920), and they've done one every summer since. For one shining moment late in Act One, when a youth chorus of 70 local kids augmented the "Be My Guest" production number, viewers could glimpse the tradition of St. Louis theater -- past, present and future -- all on one stage.