She’s played everyone from The Iron Lady’s Margaret Thatcher to Mary Poppins’ absentminded cousin Topsy, and now Meryl Streep is taking on a new challenge: Her daughter Mamie Gummer recently gave birth to a son, making Meryl a real-life grandma. “I’m going to go out and ruin Mamie’s life,” Meryl recently joked to Interview magazine. “I specialize in unsolicited advice.”

But seriously, “Meryl is over the moon,” an insider exclusively revealed to Closer Weekly in the magazine’s latest issue, on newsstands now. “She has been buying tons of adorable baby clothes and toys. She’s into organic food, so she’s getting nontoxic toys and sustainable, healthy products right down to the sheets and blankets — she’s been researching voraciously, like when she does research for a part, about what’s best for Mamie and the baby.”

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Mamie, 35, is an actress who costarred with her mother, 69, in 2015’s Ricki and the Flash. “I’ve been watching her my whole life,” Mamie said previously to the Telegraph, who had the baby boy with her fiancé, producer Mehar Sethi, 36. “Not just how she acts, but how she behaves and interacts with the world.”

For her part, Meryl once told Good Housekeeping that “motherhood and marriage is a balancing act, especially when you have a job that you consider rewarding. It’s a challenge, but the best kind of challenge.”

Like her mom, Mamie is protective of her child, whose name and photo haven’t been released to the public. “Robert Redford taught me when they were babies, ‘They are not your props,’” Meryl said of her costar in 1985’s Out of Africa. “I really admired the way he protected his family. It’s something I consciously emulated.”

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As a mother, “you have to have a lot of stamina and very good organizational skills,” Meryl previously told The Talks. “I feel like I run a business, although I haven’t one. It’s planning, planning and planning.”

She’s having a lot more fun as a grandmother. “Meryl loves babies, she loves nurturing, she loves it all,” the insider said. “That little boy is the light of her life right now.”

For more on Meryl Streep, pick up the latest issue of Closer Weekly, on newsstands now — and be sure to sign up for our newsletter for more exclusive news!