The Metropolitan Opera’s stage door, a plain entrance hidden in the tunnels of Lincoln Center, routinely welcomes star singers, orchestra musicians, stagehands, costumers and ushers. But a different bunch of visitors arrived there on a recent afternoon, carrying stuffed toy rabbits and “Frozen” backpacks.

They were children, ages 7 to 10, dressed in patent leather shoes, frilly socks and jackets decorated with dinosaurs. They were united in a common mission: to win a spot in the Met children’s chorus, a rigorous, elite training ground for young singers.

“This might be the biggest day of my life,” said Naomi Lu, 9, who admires pop singers like Taylor Swift and Katy Perry. She was knitting a lilac friendship necklace to stay calm as she waited in the lobby. “I feel nervous and excited at the same time,” she said. “You could say I’m nerv-cited.”

Singing in the shower or in a school choir is one thing. But these students, who came from across New York City and its suburbs, were vying for the chance to perform at the Met, one of the world’s grandest stages, a temple of opera that presents nearly 200 performances each year. Chorus members have a chance at roles like the angelic boys in Mozart’s “The Magic Flute”; the Parisian kids in Puccini’s “La Bohème”; or the street urchins in Bizet’s “Carmen,” to name a few.