"Have you thought about adding more to the group?" David Letterman asked the doting mom of 9-year-old twins Hazel and Phinnaeus and 6-year-old Henry on The Late Show Thursday night. "Any chance?" he asked.
"No," replied the superstar.
But Roberts, 46, is quite happy to talk about her three children, PEOPLE learned at the premiere of her new film, August: Osage County. In the film, she curses like a sailor and unleashes a string of obscenities without any regret.
But in real life, Roberts reveals she has never used foul language around her kids – except once.
"In the nine years of being a mother, I've only done that one time, and it brought all of us to a screeching halt," the Oscar winner told PEOPLE at the Weinstein Company's New York premiere of her drama on Thursday. "Everyone was like, 'What just happened?' It was horrible! I still think about it now, and I still can't believe I did that. The children just stopped, and I said, 'I am very sorry!' "
What spurred the actress – a Golden Globe and Screen Actors Guild nominee for August, who rocked the DeLeón Tequila-sponsored red carpet Thursday in a Proenza Schouler frock and Wilfredo Rosado jewelry – to say a bad word?
"It was a traffic thing," she explained. "Somebody cut me off, and I was like, 'f––!' It was dangerous. The children and I could have been hurt. Then I told them, 'Mommy is so sorry,' and we all just kept on going. We all recovered!"
Having moved on from that incident, Roberts, who is earning rave reviews for her turn opposite on-screen mom and fellow Globe nominee Meryl Streep, revels in profanity-free conversations at the dinner table with her husband of 11 years, Danny Moder, 44, and their little ones. Unlike her epic dinner-table fight with Streep in the movie, Roberts chats with her kids over a home-cooked meal.
"Our conversations are so much happier than that," she said enthusiastically. "I ask, 'How's school? Who has homework?' It's easy."