Prince William quipped about four-year-old son Louis ' behaviour as the youngster missed today's trip to Cardiff.
William and Kate's youngest child was absent from Cardiff Castle, where the couple and eldest children George and Charlotte oversaw preparations for a Jubilee celebration tonight.
Anna Jones, 53, who had been in town to pick up her wedding dress, and new friend Sandra Harris, 74, engaged the children in a lively conversation about the Jubilee.
"Where's your brother?" they wondered. "We were wondering where he was."
Prince William told them the trip would have been a bit much for the four-year-old, gesturing to the floor where he jokingly predicted Louis might have laid down.
The ladies asked the children how their first Trooping carriage ride had been, telling Princess Charlotte they had seen how she "stopped Louis waving nearly too much" in top big sister fashion.
"Is she bossy, George," they joked, to laughter.
"At least you didn't say it was mummy who was bossy," Kate told him.
The Cambridge clan then made their exit out of the castle gates, in front of a crowd of members of the public and photographers.
Charlotte appeared absorbed in admiring a bunch of tulips she had been given, tied with a ribbon.
"Thank you very much," the Duke said as they were leaving. "Good luck tonight!"
George and Charlotte helped get the Platinum Jubilee party started in Cardiff as they turned their hands to running a sound mixing deck and conducting an orchestra.
The young Cambridges made a surprise appearance at Cardiff Castle on Saturday, joining the preparation for a Jubilee concert and taking a star turn themselves.
With the encouragement of the Duke and Duchess of Cambridge, they were allowed temporary free rein of the sound and light decks for the main concert stage, looking thrilled as they changed the levels as a choir sung live before them.
The Duchess shared snippets of their own musical talents, with Prince George learning the electric guitar, Princess Charlotte practising the piano and the family all appearing to be big fans of Encanto and its catchy song "We Don't Talk About Bruno".
It is the first time the two children have made an official visit to Wales.
Prince George, eight, will one day by Prince of Wales, and - as the Duchess told one concert organiser - lived there in Anglesey when he was a small baby.
Celebrating Wales, the Duchess wore a vivid red dress from Eponine and earrings from local brand Spells of Love.
Prince Louis did not join the family for the 90-minute-long event, considered too young after his star turn at Trooping the Colour this week.
As Prince William joked to one member of the public during a walking, he could have done "anything" in the excitement, including lying down on the floor for fun.
Prince George and Princess Charlotte, seven, at times appeared shy during the event, one of very few official engagements they have done in their short royal lives.
Both politely shook hands with dignitaries and countless members of the public, introduced over and over by their parents as "this is George, and this is Charlotte".
When one man commented on their big week of Jubilee celebrations, the Duchess stroked their hair fondly and said: "They've been busy!"
Sticking close to their parents and often holding hands with either Duke of Duchess, the children were dressed smartly: George as a mini-me of his father in a smart suit and brown suede shoes with no tie, and Charlotte with her neat plaits and white ankle socks.
The Duchess kept up a stream of encouragement as the children walked onto a large grass area in front of a stage, in the shadow of the castle.
"That was good!" she said of the rehearsals on stage.
As the Welsh Pops Orchestra struck up with the catching refrain of "We Don't Talk About Bruno", the Duchess started dancing on the spot in her high heels
But as the family was invited into a small tech tent, helped on stage with the crew, George and Charlotte were immediately absorbed in the array of buttons before them.
A clearly thrilled George tried his hand at sliding the sound mixing buttons to vary which performers were loudest, listening through headphones as he was shown how to isolate one microphone.
At one point, the family laughed as someone - naming no names - caused a screech of feedback, with George jumping slightly and giggling.
"What does this do?" he asked Chris Hewitt, the front of house sound engineer who was showing him the ropes.
Afterwards, Mr Hewitt said of George's efforts: "He was doing a bigger gig than I was at his age."
At the lighting deck, Charlotte joined in with the questions, watching how she could wield the power to turn stage lights on and off.
When it was time to move on, Prince William reminded his children: "Say thank you! We'll go before we break anything."
Charlotte gave a final flourish of lighting before the family left, walking across the grass to the main stage where they would be introduced to performers from the Wales Youth Choir for Good and Welsh Pops Orchestra.
The Duchess spoke to string players from the orchestra, telling them she would have loved to learn a string intrument but played the flute instead.
She told cellist Katy Cox that the children were beginning their own musical journeys, learning electric guitar and piano.
The family were then invited to try conducting the orchestra, with expert conductor John Quirk handing over a baton which appeared to be a pencil.
The children initially shook their heads, unsure of what to do, but the Duke led the way in an enthusiastic amateur flourish he joked only served to proved the orchestra - which didn't miss a beat - quite rightly "don't listen to him anyway".
Enticed by a medley from children's film Encanto, Charlotte were on to have a turn, looking utterly delighted with the orchestra's efforts under her baton.
"I see you do requests, which is great," said William, thanking them.
The Duchess did her own spot of conducting, to Tom Jones' anthem Delilah, dancing along to the music.
The family were then introduced to Welsh weatherman Owain Wynn Evans, who rose to fame in lockdown when he drummed along to the BBC news theme tune.
He gave the children a short demo, showing them his technique.
The Duke pointed out his glamorous gold jacket, trousers with a shiny gold seam, and sparkly shoes in admiration.
"You need some in your hair too," Prince George said.
The family were also introduced to Bonnie Tyler, who is due to sing in the Jubilee concert in Cardiff this evening.
They then made their way down a short line-up including Aled Jones. Congratulated on how well the children had done, the Duchess agreed: "It [the stage] is quite overwhelming isn't it, it's so big."
Prince George was introduced to a Welsh schoolgirl who shared the same birthday as him, July 22.
"On July 24 last year, we were already making a birthday list for next year so I hope you've done the same," William told her, smiling.
As the family walked off stage together, the Duke told George and Charlotte: "Well done! Good conducting."
They then spent time with Rubicon Dance Company, who were rehearsing on a small makeshift stage on the grass, and clapped as teenagers and young children showed off their moves.
After a group dance from the children, William asked them: "Who can jump the highest? Do any of you like singing too?"
The Duchess wondered: "Are you going on stage later? It might be a bit late for you."
The Duke, Duchess and their children were next introduced to representatives of homelessness charity Shelter Cymru, having a serious conversation about the problem in Wales as Bonnie Tyler rehearsed Total Eclipse of the Heart in the background.
"I've never had to shout over Bonnie Tyler before," the Duke said afterwards. "She's a bit of a hero."
Artist Jason Wyburn presented them with two large images, one of the Queen and one of the Duke and Duchess, made up from a mosaic of tiny pictures of the Royal Family.
Princess Charlotte leaned in closely to peer at the individual thumbnail images.
As the engagement came to a close, the Duke and Duchess stayed around 15 minutes longer than planned for a walkabout with a small crowd of members of the public inside Cardiff Castle.
They met groups of air cadets, a girls rugby team and young dancers dressed in colourful saris.
Introducing him to local scouts, the Duchess nudged Prince George to tell then if he knew any scouts, naming Bear Grylls.
Asked by cadets about Trooping the Colour, the Duke said: "Did you see the flypast? These two were amazed, even I was quite overawed."
The children coped faultlessly with the experience of the walkabout, with camera phone pointing at them from every direction and cries of "Prince George, can I shake your hand?" and "look at Princess Charlotte, isn't she so pretty!"
They made a brief stop to say hello to The Royal Welsh goat, Shenkin IV, and accepted bouquets of flowers from well-wishers.
