Idina Menzel and her sister Cara Mentzel talk to PEOPLE about their debut kids' book, Loud Mouse, out Sept. 27
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Idina Menzel and sister Cara co-author kids' book Loud Mouse
Cara Mentzel and Idina Menzel
| Credit: Cara Mentzel

Idina Menzel was celebrated for her powerful voice even as a child, but her talent came with its own set of insecurities.

This experience inspired the Broadway singer to write her first children's book with her sister Cara Mentzel, Loud Mouse — a story that empowers young readers to embrace the "light" inside them.

"That's something that I've dealt with ever since I was a little girl: wanting to step into my light, my spotlight to shine, to embrace this voice that I had, but also not wanting to alienate the people around me," says Idina, 50, in a joint interview with Cara about the book, which will be published by Disney Hyperion on Sept. 27.

The Frozen actress explains that she never wanted to "seem like I was too much for anyone, that I was showing off, to bring attention to myself."

"As I've grown older, I realize that we're doing a disservice to the world when we hide our talents and our gifts," Idina says. "And when I finally understood that is when I finally found joy in what I was doing and acceptance. So, that was when I called Cara one night and said, 'Now go write a book about that.' "

And Cara did just that.

Loud Mouse follows a young mouse named Dee, who loves to sing and has a very supportive sister. When she performs in front of her class for the first time, something magical happens.

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Loud Mouse
Credit: Disney

"I think the message we want us to send is that we have a job to shine in whatever way we can and not to shy away from that," says 47-year-old Cara, a former elementary school teacher who released her memoir about growing up with Idina, Voice Lessons: A Sisters Story, in 2017. "And that's not a selfish thing, that's not a conceited thing, that's what we're here to do."

Idina says she put off writing a kids' book for a while until she felt she could create it from an authentic place.

"Nothing against some of my peers, but I didn't want to do a celebrity-driven kids' book for the sake of doing it," she says. "I wanted it to come from a real place and I wasn't sure what that was yet."

Knowing that she could partner with her sister, whom she'd previously collaborated with on other projects, solidified her decision, Idina explains.

"I knew that she would help me interpret whatever it was that I wanted to do because she knows me," says Idina. "She's a great writer, but also a teacher of kids this age. [Cara] has an understanding of children's literature from a literacy standpoint, from a content standpoint— she's a good artist, everything."

The actress is thrilled with the finished story. "With Cara, it just was so right on and better than I could have imagined because I just love her metaphors and poetry that she uses," says Idina. "And I think there's a sophistication, which I hope parents will appreciate because it lives in two levels."

Idina Menzel
Idina Menzel
| Credit: Ricky Middlesworth/Disney

Idina says that the creative process for Loud Mouse has been one of the "most organic" of her career. The sisters even presented their own drawings when they pitched the manuscript to publishing companies.

"They were both horrible," Cara explains with a laugh. Together, they chose the illustrator, Jaclyn Sinquett.

"We chose the one who made us the most attractive as mice," teases Cara. "Just kidding."

They loved Sinquett's work because they wanted the illustrations to be artistic and "beautiful, but also be funny," says Idina.

Cara also gave her sister advice about the melody that Idina is recording for the book.

"Cara said, 'Just don't write a song that people like me and moms and teachers can't sing,' " Idina recalls.

Their kids have also given feedback during their writing process. Cara's children are adults and would tell her when she needed to make the storyline more "organic." She also shared the book with Idina's 12-year-old son, Walker Nathaniel, whom Idina shares with ex-husband Taye Diggs.

Cara Mentzel
Cara Mentzel
| Credit: Sherial Starr

At the beginning of the pandemic, Idina organized with other local moms to start a school pod at a house in Los Angeles, with Cara as the teacher. Walker took classes with four of his friends for a year.

"It was this amazing, life-altering thing that we were very fortunate to be able to do, to take our children and drop them off in Cara's lap every day," says Idina. "I think my son actually will listen to Cara, Aunt Cara as he says, much more than me. So he's heard the book through Cara. I wouldn't dare try to get him to sit down and read it right now. He's like, he's way too cool for school."

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Loud Mouse is Idina and Cara's first writing collaboration — and it won't be the last. They're already discussing ideas for a second book, one that captures more of Cara's personality.

"We're each other's biggest fans," says Idina. "Me, being the big sister, that took up a lot of space. People talked about my voice, I got the solos in the choir and all the family was coming to see me. And then little Cara was always trying to figure out where to be."

"Not everybody is an Idina Menzel, [who] has the issue of being so big... and then wondering how to take up that space with grace," adds Cara. "Some people don't know how they shine, or what that is right away, and they're searching for it. So, in a follow-up, we wanted to speak to kids like that. That were more like me."

Loud Mouse hits bookstores this fall.