"It was a lovely, jolly affair," a palace source tells PEOPLE

By Simon Perry
Updated October 05, 2016 06:19 PM
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Annie Leibovitz

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Little royals on parade!

Prince George, Princess Charlotte and all the Queen’s youngest children and great-grandchildren posed for a new portrait released Wednesday in honor of Queen Elizabeth‘s 90th birthday on Thursday.

The arresting portrait, snapped in the Green Drawing Room of Windsor Castle, is one of three taken at the Queen’s famed residence on March 28 by famed celebrity photographer Annie Leibovitz.

“It was a lovely jolly affair,” a palace source tells PEOPLE. “It was very relaxed.”

The source says there was no “particular logic” to featuring only the littlest seven of the Queen’s eight grandchildren and five great-grandchildren in the picture: “It was felt to have just the younger ones.”

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Occupying a place of honor in the monarch’s lap – befitting a tradition of royal portraiture – is nearly 1-year-old Princess Charlotte. (With their matching pink cardigans and blue eyes, the two make a strong bid for royal twinning!)

Meanwhile, Prince George – who took his own place of honor in another royal portrait released on Tuesday – sports his signature Amaia knee socks as he stands between his cousins and VIP playmates, 5-year-old Savannah and 4-year-old Isla, the daughters of Peter Phillips. (Peter’s mom is the Queen’s only daughter, Princess Anne.)

The adorable blonde toddler holding the Queen’s famous Launer purse? 2-year-old Mia Tindall, the daughter of Peter’s sister Zara and her husband Mike Tindall.

“She just did it,” a palace source tells PEOPLE. “She just picked it up – it wasn t staged. It was completely spontaneous.”

Looking after the little ones are the Queen’s youngest grandchildren, Lady Louise, 12, and James, Viscount Severn, 8, the children of the Queen’s youngest son, Prince Edward, and his wife Sophie.

In addition to the multi-generational photo, the palace also released two other portraits taken by Leibovitz: The Queen with her beloved dogs Willow, Vulcan, Candy and Holly – she poses with them on the castle’s East Terrace – and a photo taken in the White Drawing Room at the castle of the Queen and her daughter Anne, 65.