Prince William and Duchess Catherine will travel to Scotland with their children in the coming days.
Prince William and Duchess Catherine
The Duke and Duchess of Cambridge will take Prince George, two and four-month-old Princess Charlotte to Queen Elizabeth's Balmoral estate where they will help the monarch celebrate becoming Britain's longest-reigning monarch on September 9.
William and Catherine, both 33, are expected to join the 89-year-old queen and other relatives for a special dinner to mark the milestone.
A source told PEOPLE: "It's a milestone day for the queen, but for her it is 'business as usual' and she has her engagements during the day.
"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."
The queen will take over from her great-great grandmother Queen Victoria as Britain's longest-serving monarch.
The visit will give Princess Charlotte a chance to wear the tartan pinafore she received as a gift from a bagpipe band recently.
The royal was given the plaid dress by a member of the Sons of Scotland Pipe Band, who performed for her great-grandmother, Queen Elizabeth II, at Balmoral Castle last month.
Bethany Bisaillion played traditional pipe music for the queen and Prince Philip, 94, their daughter-in-law Sophie Wessex, 50, and her daughter Lady Louise, aged 11, and said she was "emotional" when she handed the gift over.
She gushed: "I was getting a little emotional. I had to bite my lip and be stoic!
"We hope that she gets it in time that she doesn't grow out of it!"
The little dress features a maple design similar to items made for Prince George and Princess Catherine and the band also made a waistcoat for Prince William, which they presented to the royals on previous visits.
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