Go Inside Princess Diana's Childhood Home with Her Sister-in-Law, Countess Spencer (There's a Bouncy Castle in the Dining Room!)

The extraordinary home – which sleeps 60, is situated on a 550-acre park and has nearly 100 fireplaces – is the ultimate family house

Photo: Simon Emmett/CLM

When Karen Spencer became a countess in 2011 after marrying Charles Spencer, Princess Diana‘s brother and the 9th Earl Spencer, she moved into Althorp, her new family’s 508-year-old English estate – and started making some changes.

Among them: the addition of a bouncy castle to the state dining room.

“A lot of people are quite surprised by it,” Spencer, 44, tells PEOPLE in an exclusive interview in this week’s issue. “The one problem is that the children get terribly upset when we actually want to use the state dining room for. . . well, dining.”

Now Countess Spencer – whose nonprofit, Whole Child International, helps change the lives of children living in Third World orphanages – is opening the estate alongside her husband in a fundraising effort for Whole Child.

The couple will host 20 benefactors at Althorp for a traditional English country weekend. (Starting price: $25,000.) In addition, they will also offer a similar weekend at the estate for six small donors, including those who solicit donations from the biggest number of friends, family and others – along with the winners of video/essay contest.

For “the lady of the house,” as Charles describes her role there, the extraordinary home – which sleeps 60, is situated on a 550-acre park and has nearly 100 fireplaces – is the ultimate family house.

“I’m passionate about using the whole thing,” says the mom of 3-year-old Charlotte Diana as well as two daughters from her first marriage, Emma, 18, and Kate, 15. (She’s also a stepmom to Charles’s six kids from two previous marriages.) “It’s got such great energy. The very first time I walked into it, I just felt instantly at home.”

The historic residence, where Diana moved when she was 14, holds countless memories of the princess.

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Spencer’s youngest daughter enjoys rowboat rides to “visit Auntie Diana” on the tiny island where the beloved royal is buried. The grounds and interior are currently undergoing an elaborate renovation – including a new memorial at Diana’s grave.

“She was Charles’s closest family member,” says Spencer. “That loss is still palpable in my husband.”

Formerly married to Hollywood producer Mark Gordon, Spencer founded Whole Child in 2004 after researching a relationship-based approach between kids and caregivers.

“That’s the core piece in the early years,” she says. “And it’s completely doable.”

Charles, 52, says his wife’s passion often reminds him of his late sister.

“Like Diana, she gets her hands dirty with causes that other people might quickly walk away from,” he tells PEOPLE. “What my sister did for people with HIV/AIDS, the homeless and those suffering from leprosy, Karen is doing: fighting for orphans and abandoned kids.”

• With reporting by NINA BIDDLE