Nicole Kidman and Beyoncé appeared to defy a longstanding protocol at the Met Gala by bringing their respective teenage daughters to the red carpet . In 2018, Vogue confirmed that under-18s weren’t allowed to attend the fashion extravaganza, after 15-year-old Maddie Ziegler told The Hollywood Reporter she wasn’t “old enough” to go. Met Gala organizers told the outlet that “it’s not an appropriate event for people under 18.” However, Met Gala co-chairs Beyoncé and Kidman were the exceptions to the rule last night. The Moulin Rouge! star walked the blue carpet in a deep cherry red gown with a feathered hip trim by Chanel, while holding hands with her 17-year-old daughter, Sunday Rose, whom she shares with ex-husband Keith Urban. At the event, where the dress code this year was “Fashion is Art,” Sunday wore a Dior baby-pink set — featuring a strapless bubble top and a sheer maxi skirt — embroidered with purple and blue flowers.…