" 'When you're in doubt, don't,' " Kathryn Hahn tells PEOPLE exclusively about one of her own mother's best pieces of parenting advice

They may play Bad Moms on screen, but in real life, they’re nailing parenthood — with a little advice from their own mothers.

A Bad Moms Christmas stars Mila Kunis, Kristen Bell and Kathryn Hahn open up in this week’s issue of PEOPLE about their real lives as moms, and reveal that they’ve gotten a lot of tips from the women who raised them.

“I was holding [daughter Wyatt Isabelle, now 3], she was maybe two days old. And my mom was just looking at her, and she goes, ‘Oh, would you look at that? She’s peeing,’ ” says Kunis, who also shares 11-month-old son Dimitri Portwood with husband Ashton Kutcher. “And I was like, ‘How do you know?’ ”

“Because of her face. Her nose flared, and this little part turned white. And my mom was like, ‘You can just tell.’ And I was like, ‘Clearly a mother,’ “ continues the star, 34, who adds that she was raised on cloth diapers. “Never would I have ever known that. So from that point on, I s— you not, I knew exactly when my kids were peeing.”

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Mila Kunis, Kristen Bell and Kathryn Hahn

Mila Kunis, Kristen Bell and Kathryn Hahn

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Hahn — mom to daughter Mae, 8, and son Leonard, 10 — says her own mother gave her two pieces of advice that she values highly when parenting alongside husband Ethan Sandler.

“One was, ‘The days are long and the years are short,’ which I remember,” says the 44-year-old actress. “Another one was, ‘When you’re in doubt, don’t.’ Which is just good life advice, I think. If you’re waffling, there’s a reason.”

Adds Bell, 37, “My mom said, which is what my grammy used to say a lot, ‘You can be anything, but you can’t be everything.’ ”

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Credit: Jim Wright

Mila Kunis, Kristen Bell and Kathryn Hahn

The women have their own pieces of advice based on lessons they’ve learned not only in childcare, but in sharing parental duties with their husbands.

“You have to say, ‘I am feeling overwhelmed. I can’t do this by myself, I need help.’ You always have to start with ‘I’ statements,” explains Bell, who shares daughters Delta, 3 next month, and Lincoln, 4½, with actor husband Dax Shepard.

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“And if your partner loves you — which they should, because you guys got married — then they will see the fact that you are overwhelmed and want to meet you in the middle,” adds the Veronica Mars alum. “But it is really a lot about how you say it.”

A Bad Moms Christmas — also starring Susan Sarandon, Christine Baranski and Justin Hartley — is in theaters now.

For more details into the parenting lives of the cast of A Bad Moms Christmas, pick up the latest issue of PEOPLE, on newsstands now.