Their dad’s would be confidential and offer advice, tucked away in the top drawer of the Resolute Desk in the Oval Office, where incoming President Barack Obama would find it on his first day of work as leader of the free world. Now a White House tradition, this had been done by every sitting president since Ronald Reagan.

President Obama walks down the White House Colannade with his two daughters in this undated White House photo.

And so as their father prepared to pass the torch, the Bush girls decided they had advice to offer, too – not for the new president, but for his young daughters, Sasha and Malia.

From one pair of first daughters to another, they titled it “Playing House in the White House.”

Barbara and Jenna, then 27-years-old, told Sasha, 7, and Malia, 10, to surround themselves with “loyal friends,” to cherish their pets in times when they’ll need “the quiet comfort that only animals can provide,” to slide down the banister of the solarium and play sardines on the White House lawn.

Most importantly, they said, “remember who your dad really is.”

Eight years later, with just a week until Sasha and Malia leave the White House, the Bush sisters, now 35, have written a second letter – this time with advice not about living inside the White House, but out of it.

“We have watched you grow from girls to impressive young women with grace and ease. And through it all you had each other. Just like we did,” they wrote in the letter, published first online by Time magazine. “Now you are about to join another rarified club, one of former First Children – a position you didn’t seek and one with no guidelines. But you have so much to look forward to. You will be writing the story of your lives, beyond the shadow of your famous parents, yet you will always carry with you the experiences of the past eight years.”

Americans have an odd fascination with the idea of first daughters. It has inspired a handful of popular movie narratives – Katie Holmes’s “First Daughter,” the Disney classic “My Date with the President’s Daughter” and “Chasing Liberty” starring Mandy Moore – and was a major plotline in Aaron Sorkin’s popular TV series “The West Wing,” which often wove in the fictional president’s complicated relationship with his three daughters.

The scrutiny these fictional first daughters faced is not unlike the pressure the real ones encounter, and for the past quarter century, Americans have had only female first children in the White House – Chelsea Clinton, the Bush twins, Malia and Sasha Obama.

And perhaps no first children were confronted with the harsh reality of having a dad in chief as much as Jenna and Barbara Bush, whose father was in the White House during their college years and whose antics with underage drinking drew intense publicity.

The sisters made note of that in their letter to the Obama sisters.

“Enjoy college. As most of the world knows, we did,” they wrote. “And you won’t have the weight of the world on your young shoulders anymore.”

In 2014, a Hill staffer for a Republican congressman resigned amid widespread backlash after she criticized on social media the appearance of Sasha and Malia during the televised traditional turkey pardon on Thanksgiving.

“Dear Sasha and Malia, I get you’re both in those awful teen years, but you’re a part of the First Family, try showing a little class,” the staffer wrote at the time. “Rise to the occasion. Act like being in the White House matters to you. Dress like you deserve respect, not a spot at a bar.”

Jenna Bush Hager, who kept her family name after she was married, defended the Obama girls after the incident, saying she felt “fiercely protective of them.”

That sentiment was obvious in the letter she and her sister penned Thursday.

The Bush twins encouraged Malia and Sasha to keep in contact with the people that made their stay in the White House feel normal, including their Secret Service agents, and challenged them to use their political and diplomatic experiences as first daughters for the greater good.

“You have lived through the unbelievable pressure of the White House,” they closed the letter. “You have listened to harsh criticism of your parents by people who had never even met them. You stood by as your precious parents were reduced to headlines. Your parents, who put you first and who not only showed you but gave you the world.

“As always, they will be rooting for you as you begin your next chapter. And so will we.”

Jenna Bush Hager is a correspondent for the “Today” show on NBC and her sister, Barbara, is the CEO and co-founder of Global Health Corps, a nonprofit focused on the global health equity movement.