Ordinarily actresses don't take kindly to being upstaged, but charity was the order of the day in Beverly Hills at the mid-March mother-daughter fashion fund raiser for the Young Musicians Foundation. Sartorially splendid moms such as Barbara Rush, dancer Elaine Joyce and Barbara Rickles smiled indulgently as their offspring stole the runway show of I. Magnin clothes at the Beverly Hilton. Drew Barrymore
, 8, waltzed out drenched in $750,000 in diamonds and pearls borrowed from a Beverly Hills jeweler. Crying Caitlin Cassidy, 15 months (her dad is Shaun), had to be quieted by grandma Shirley Jones after a bout with stage fright. And Samantha Rader, 3½, upstaged her mother, Sally Struthers. "Now you stay right there, it's my turn," ordered Samantha, as she strutted out in bloomers for a solo run. "The only thing that's worrying me," confided Florence Henderson's daughter Lizzie Bernstein, 17, "is stepping out of my shoes on the runway." In all, $15,000 was raised for the foundation, which helped launch the careers of, among others, conductor Michael Tilson Thomas and pianist Misha Dichter.