Passengers on a Scotland bound flight were surprised to spot Kate flying economy along with Charlotte and Louis.
It is believed they were en route to Balmoral where the Queen is spending her annual summer break in the Scottish Highlands.
The Duchess of Cambridge, 40, was spotted with Charlotte, seven, and four-year-old Louis, as well as their nanny.
Prince William, 41, reportedly travelled separately with their eldest child Prince George, nine.
A surprised member of the public shared a video on TikTok of Kate disembarking the flight, reports the Express.
The social media user said: "On a flight to Scotland, Kate Middleton, both kids, nanny and security 2 rows in front of me.
"Could get a private jet or helicopter but keeping it simple, flying economy," added the user.
"She even got up halfway through the flight to get her daughter's iPad.
"And yes she was stunning, even gave me a smile while looking back at row behind her to talk to her son.
"No one on board took any photos or videos.
"Just a mum taking her children on holiday to see grandmother, but there was a buzz on board."
Kate was seen wearing a midi skirt, khaki blazer and cream shoulder bag. Charlotte wore a floral dress, while Louis was smart in navy shorts and a blue shirt.
The Queen traditionally spends the summer at her Scottish retreat where she is joined by other members of the Royal Family.
The Cambridges' visit comes ahead of their reported move to Windsor.
The family-of-five are understood to be relocating from Kensington Palace to Adelaide Cottage.
The move will mean they are closer to the monarch, 96, who is now mainly based at Windsor Castle.
They will also be closer to Kate's parents Carole and Michael Middleton, who live in the Berkshire village of Bucklebury.
George and Charlotte are expected to enroll at a school near Windsor.
Kensington Palace will remain the Cambridges' London home and they also have their Anmer Hall country retreat in Norfolk.
And according to royal author Omid Scobie, William and Kate realise that Windsor is a better place for George, Charlotte and Louis to grow up - despite their original plan.
Writing in his column for Yahoo, he said: "Though London was originally the dream, it became increasingly apparent to the duke and duchess that living in the middle of Kensington Gardens may not be the best place for Prince George, nine, Princess Charlotte, seven, and Prince Louis, four, as they grow up."
He also says a source told him that the Cambridges had thought about moving permanently to Anmer Hall in Norfolk - but it is too far from London.
He quotes a source as saying: "They thought about moving to [their home in] Norfolk, but as active senior working royals they could never be that far away from London, so that’s where Windsor came into the picture."
