Prince William was caught making a sweet gesture to Louis ahead of the prince's first day at school.
The Cambridges arrived together in one car as William, 40, 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, with future Queen Kate holding 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.
The school's headmaster Jonathan Perry met 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!"
"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.
Prince George, Princess Charlotte and Prince Louis told how they were "excited" to start their new school together, as they were welcomed with a firm handshake from their headmaster.
The three Cambridge children, George who is nine, Charlotte, seven and Louis, four, have enrolled in the prestigious Lambrook school in Berkshire, after the recent family move out of London to Windsor.
She remarked on the unexpectedly lovely weather, saying it has been nice every time they had visited the school.
It is the first time all three siblings have attended school together, with Louis starting his education in pre-prep and his older siblings joining the prep as new starters.
The family have settled into their new home down the road at Adelaide Cottage ready for the start of the school year and to be closer to the Queen who now lives permanently at Windsor Castle.
A royal source said the Cambridge children were particularly excited to all be going to the same school for the first time, adding: “They're really excited about starting a new school together, and I think mum and dad are too.”
Yesterday, the children 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.
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 Cambridges spent around 90 minutes at the school, meeting fellow parents and other children also starting this year.
The full school starts its new term tomorrow.
The future king and queen had thought “long and hard” over the issue with one palace insider saying their “overriding focus being to provide as “normal” a life as possible for their children before beginning their own royal lives.”
The Cambridges will also fork out eye watering fees for the children’s new school, spending in excess of £53,000 a year on their private education.
Fees cost £4,389 a term for Prince Louis, who is four, £6,448 per term for Charlotte, seven, and £6,999 per term for nine year old George, until pupils are 13.
The bill amounts to £53,508's worth of fees in 2021-2022.
The school boasts its own 9-hold golf course set in 52 acres of idyllic Berkshire countryside.
Lambrook School suggests it gives its pupils "feathers to fly" and a "delicious sense of freedom".
The Good Schools Guide describes it as a "classic prep school" with a "heart of gold", and tells of how youngsters get to "run and run" in the vast grounds with "total freedom to explore, provided you've got your wellies on".
Lambrook boasts of "first-class teaching and superb facilities" which include a 25-metre swimming pool, a nine-hole golf course, an astroturf, hard courts, a squash court, cricket and other sports pitches on the outskirts of Bracknell, a 20-minute drive from the Queen’s Windsor Castle home.
It has a Diamond Jubilee performing arts studio, dance studio and sports hall, and a new £6 million Queen's Building for ICT and academic learning.
Lambrook offers weekly and flexi-boarding for boys and girls aged seven onwards, with the Duke and Duchess of Cambridge having the option to let George and Charlotte stay as little as one night a week on an ad-hoc basis, with the sleepovers booked online.
George and Charlotte will be day pupils for now while Louis, who will be in reception, will enjoy "Forest Fridays" and be "taken on a journey of discovery in the beautiful outdoors", the school's prospectus says, mirroring the Duchess of Cambridge's philosophy of the importance of outdoor play and spending time in nature.
The family’s downsizing to the smaller Adelaide Cottage means William and Kate's full-time nanny Maria Teresa Turrion Borrallo will live elsewhere for the first time, as will other staff including the housekeeper and the chef.
