She shares two daughters with her husband, Keith Urban.
The 51-year-old actress stops by The Ellen DeGeneres Show on Tuesday where she gives up one major secret about the hit series' upcoming sophomore run.
"[My kids] were in as extras on Big Little Lies," she reveals to the host of her two daughters with husband Keith Urban, Sunday, 10, and Faith, 7. "I've never told anyone. There I am now being a big mouth."
Though she let that one bit of info slip, Kidman, who also serves as a producer on the show, is generally good at keeping the plot under wraps. When it comes to her and Reese Witherspoon -- her co-star and co-producer on the series -- they can have some trouble staying mum.
"Probably the two of us have a bit of a big mouth. So we have to watch it," Kidman quips, before defending Witherspoon by saying, "She's a very good secret keeper!"
"She's told me some things and I'm trying to keep the secret," DeGeneres, 60, disagrees. "I'm not going to tell you because I'm good at keeping secrets, but she's told me the entire plot line for this next season."
Kidman, it turns out, is guilty of sharing the plot too! The Aquaman star spilled the beans to 51-year-old Urban, her husband of more than 10 years.
"I've told my husband the entire plot line, but you're allowed to do that," she reasons. "Except he doesn't know the Aquaman story line. The story is on even bigger lock down. Those comic book films are like oh my god."
Kidman is set to play Queen Atlanna in the film, which is due to release Dec. 21. That role, she says, has made her a major hit with her kids. "That's the only thing that gives me any credibility with them in terms of my career," she jokes.
While her kids are impressed the film role, many others are preemptively impressed with season two of the HBO series, specifically the casting of Meryl Streep. The 69-year-old actress is set to star as Kidman's mother-in-law who moves in with her at the beginning of the season.
"We got lucky," Kidman gushes of Streep's casting. "[Streep said,] 'I want to be in the coven.' And she's definitely in it now."
"I think all of us being together was so beautiful," Kidman said at the time. "We've now worked together well over a couple of years and so the bonds between all of us is just deeper and stronger and richer. I miss them. And then to have brought Meryl into that equation, I mean, we would go out all the time! We talk, we help each other. It's just a very strong connection and that's rare, you know. That you get that amount of time, particularly screen time together, and then personal time together."
Watch the video below for more on the upcoming season:
Reese Witherspoon Brags About Hitting Meryl Streep With Ice Cream in 'Big Little Lies' Scene: 'I Nailed It'