Mario Testino /Art Partner
September 01, 2015 04:15 PM

Looks like bonnie Princess Charlotte will soon have the perfect setting to wear her new plaid pinafore!

PEOPLE understands that the 4-month-old royal is set for her first trip to Scotland in the coming days, as she and parents Prince William and Princess Kate head to Queen Elizabeth’s highlands estate, Balmoral.

That means they will likely be on hand to celebrate with the Queen as she passes her great-great grandmother Queen Victoria’s record-breaking milestone as Britain’s longest-reigning monarch on September 9.

It’s expected that William and Kate, both 33, will join the Queen, 89, for dinner at the historic castle that evening.

Neither the couple’s office at Kensington Palace or the Queen’s at Buckingham Palace would confirm the dinner details. “It’s a milestone day for the Queen, but for her it is ‘business as usual’ and she has her engagements during the day,” a royal source tells PEOPLE. “Any dinner will be treated like every other evening, and we won’t be briefing on who the Queen is having dinner with. That won’t change because of the significance of the day.”

Kate hasn’t been seen much in public since she gave birth to Charlotte in May (limiting herself to just one royal engagement in July), with royal sources revealing to PEOPLE in the spring that she would take a longer time out than she did with 2-year-old Prince George.

But aides are planning ahead and say that the couple will have a foreign tour at some stage next year, and they’re expected to help the Queen host the Chinese president and his wife in London in October.

Although summer visits to Balmoral – where shooting, strolling and relaxing are the leisurely order of the day – are almost compulsory for the royal family, Prince Charles and his wife Camilla will not be there on the Queen’s big day (but they did pay a visit to Birkhall, the home where they stay on the estate, earlier this summer, and are likely to celebrate with her again soon).

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Charles will be in Scotland but on a previous engagement at Dumfries House, the historic home he help save, while Camilla, Duchess of Cornwall, has her own engagement in London to mark the 60th anniversary of Britain’s ITV.

“The Prince of Wales and The Duchess of Cornwall have long- standing commitments on September 9th,” a Clarence House spokesperson tells PEOPLE, “and there is very much a sense of business as usual on this day for all members of the royal family.”

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