For the man responsible for some of the most complex melodies and intricate lyrics in the musical theater, a simple chorus of “Happy Birthday” would never do.Instead, to mark his 75th birthday, Stephen Sondheim was revered by everyone from Harvey Fierstein singing “Rose’s Turn” in character as Tevye, to Dame Edna Everage in a frock made of feathers from the “rare Australian purple emu,” warbling through a dramatic “Losing My Mind.” Those divas and a bulging roster of star performers were part of the “Children and Art” tribute Monday at Gotham’s New Amsterdam Theater. Event was a benefit for Young Playwrights, the org founded by Sondheim and devoted to writers aged 18 or younger. “And so the kid is 75 years old,” said longtime Sondheim collaborator Harold Prince. “From wunderkind to veteran to legend, all in the blink of an eye.” “I can’t believe you’re 75,” added Dame Edna. “I always thought you were a spunky, feisty 83 with wonderful periods of lucidity.” Among speakers were Angela Lansbury, Cynthia Nixon, Whoopi Goldberg and Kenneth Lonergan. Following presentation of a cake to Sondheim, guests segued to supper at the Four Seasons to usher in the composer’s actual birthday at midnight.