The royal children chorused “yes” when asked if they were excited to start their new school, as a proud Prince William ushered “all the gang” up the steps.
All three will be at the same school for the first time, with Prince Louis, four, starting his education in pre-prep at Lambrook, and his nine-year-old brother and seven-year-old sister joining the prep as new starters.
The family have settled in to their new home down the road at Adelaide Cottage, Windsor, ready for the start of the school year.
On Wednesday they joined all new pupils at the school for a settling-in session lasting around 90 minutes, in which the whole family were welcomed in for refreshments to meet their new friends.
The Cambridges arrived a little early, allowing the press to mark the milestone of the children’s first day with a short, carefully-controlled moment in front of a camera.
One reporter, one photographer and one television camera were permitted to observe the event under the supervision of one Kensington Palace press officer, who had coordinated with the Duke, Duchess and the school. Those present were asked not to film on their mobile phones, to make sure the children were not faced with screens.
Around half a dozen protection officers dressed discretely kept carefully out of sight, with no other members of the media permitted to attend in order to keep the afternoon calm and relaxed for the children.
Like other families, the Cambridges arrived together in one car. The Duke drove, while the Duchess was seen waving from the passenger side to the school’s headmaster as they arrived on site.
After parking, the family of five walked together in a line towards the school.
Kate held hands with George and Louis, while William walked hand-in-hand with Charlotte.
Conscious of his youngest son’s very first day of school, William was seen stroking Louis’ hair reassuringly as they walked into sight of the entrance.
The Duke and Duchess could be heard talking to the children and laughing, in the final moments of preparing the siblings for their new adventure.
Prince George and Prince Louis both wore the smart checked shirt of the school uniform, while Charlotte wore the blue summer dress.
They managed the walk during a gap in the rain, which later became torrential as other parents arrived.
The school’s headmaster Jonathan Perry was waiting for the Cambridges at the entrance, stepping forward to greet them.
The Duchess, who appeared to know Mr Perry and his wife well after the long process of choosing the right school, greeted them like an old friend, saying: “Nice to see you, Mr Perry!”
She remarked on the unexpectedly lovely weather, saying it has been nice every time they had visited the school.
“Welcome to Lambrook,” the headmaster told the children. “It’s lovely to have you with us. We’re very excited for the year ahead.”
Shaking them each by the hand in turn, he asked them: “Are you excited?”
All three piped up “yes!”
William said “We’re looking forward to it”, adding the children had “lots of questions”.
As they walked up the steps into school together, Mrs Perry - waiting in the doorway - said: “Welcome back to Lambrook.”
“With all the gang!” William replied.
The Cambridges were due to spend around 90 minutes at the school, meeting fellow parents and any children also starting this year.
While parents enjoyed refreshments together, the pupils were taken to their new classrooms to meet their teachers again, familiaries themselves with the room and ask any questions they have.
The full school starts its new term on Thursday.
A source said the Cambridge children were particularly excited to all be going to the same school for the first time.
“They’re really excited about starting a new school together, and I think mum and dad are too,” said the source.
William and Kate made the switch to their new four-bedroom home in a bid to give their offspring a more normal life, with more freedom away from the goldfish bowl of Kensington Palace in the heart of London.
Outdoorsy and nurturing prep school Lambrook, where fees are costing William and Kate more than £53,000 a year, has been described by parents as “magical”.
It boasts of giving its pupils “feathers to fly” and features 52 acres of grounds and state of the art buildings and resources.
The Good Schools Guide describes it as a “classic prep school” with a “heart of gold”, and says youngsters get to “run and run” in the vast grounds with “total freedom to explore, provided you’ve got your wellies on”.
The children will be day pupils, but Lambrook also offers weekly and occasional boarding.