MANCHESTER, England — It was that moment after the music ends. The pop star Ariana Grande had finished the encore of her “Dangerous Woman” concert, and the shrieks of teenagers and others had subsided. The stage show was over, the arena lights had gone up, and fans were clutching pink balloons that had dropped from the rafters — souvenirs from a special night.

Lisa Conway, 49, had secretly bought tickets a month earlier and booked a room at the nearby Park Inn Hotel. It was a surprise for her 14-year-old daughter, whose favorite artist is Ms. Grande. Mother, daughter, father and son came down from Glasgow, but the father and son skipped the show for a night on the town. The concert was a bonding trip, one of those markers of adolescence, and a small, tentative step into the adult world.

Then, with the arena still tingling with the exhilaration of the music, came the explosion.

“It was meant to be a dream, not a nightmare,” Ms. Conway said Tuesday morning while eating breakfast at the hotel, trembling as she struggled to contain tears. “There were children, blood, shoes, splattered all over the floor.”

She added: “How can I explain any of this to a 14-year-old? She hasn’t said a word since she woke up from two hours’ sleep.”