According to UK media, Queen Elizabeth has overridden the rule for children while they are young.
However, once Prince George turns 12 years old, the Queen will not be able to make exceptions and he must travel separately from his father.
Similarly, Prince William cannot travel with Prince Charles and Prince Charles cannot travel with his mother, the Queen.
The Cambridges reportedly headed to Balmoral after landing in Scotland. One lucky passenger was seated near the royals and shared a clip of them disembarking the flight after sitting in economy like regular travellers.
"On a flight to Scotland, Kate Middleton, both kids, nanny and security two rows in front of me. Could get a private jet or helicopter but keeping it simple, flying economy," said TikTok user @turbom1k3y, in the video.
"She even got up half way 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," they added.
One user joked that the increase in living costs had apparently hit everyone.
"Even royal family affected by cost of living crisis," quipped one user.
