Christie Brinkley said it was a "nightmare" to put on a brave face and throw a birthday party with 30 kids amid revelations that husband allegedly had an affair with a teenager.

But the supermodel -- testifying Monday at the custody portion of her Long Island divorce trial -- said that homespun family celebrations are part of a lifestyle that means the world to her.

"That's the most important thing -- to have a big, happy family," Brinkley tearfully told the court.

Brinkley said she tried to spare her kids the messy details of the breakup, including her discovery of husband Peter Cook's Internet porn habits.

Brinkley testified that she told the children. "I think Daddy has fallen out of love with me." But she assured them they would always be "very, very loved."

In her testimony last week, Brinkley said, "My world was completely shattered. My life as I knew it had vanished," upon learning of Cook's alleged affair.

She also said that Brian Platt, a police officer and the stepfather of the 18-year-old, informed Brinkley of the affair at a 2006 graduation at Southampton High School where the model was a speaker.

He tapped her on the shoulder, she tearfully recalled, and said, "That husband of yours won't knock it off. He's having an affair with my teenage daughter."

During his testimony Wednesday, Cook said he had given his mistress, Diana Bianchi, a $300,000 payoff, had extramarital trysts in his office and Brinkley's Hamptons homes, and spent thousands of dollars on online porn.

Brinkley, 54, and Cook, 49, wed in 1996. She filed for divorce a decade later, amid the public scandal over the affair.

Report: Brinkley Visits Ex-Husband Joel

According to People magazine, Brinkley and her daughter, Alexa Ray Joel, spent the Fourth of July holiday with her ex-husband, Billy Joel, at the musician's home in Sag Harbor, N.Y.

Brinkley was among about a dozen guests at Joel's gathering.

"She was just blowing off some steam, being with family," a source told People. "It was just a relaxing little get-together. Nothing special. Just what everyone does on the Fourth."