Prince Louis had to be "tidied up" by mum the Princess of Wales as they took part in the first Trooping the Colour parade of King Charles' reign.
The pair were joined in a carriage by Queen Camilla, Prince George and Princess Charlotte for the birthday parade to travel from Buckingham Palace to Horse Guards Parade for the ceremony.
Once the show of military precision was over, the royals all piled back into the carriages to head back to the palace for the flypast, when Kate shared the sweet mum moment with her youngest son.
For the outing, Louis looked dapper in a shirt and tie with with his trademark shorts, and as they prepared to leave Horse Guards, Kate could be seen appearing to tell the five-year-old to sit up as she fussed with his tie.
Even BBC commentator Huw Edwards remarked that Kate was "tidying up" Louis before they headed back to the palace for the finale.
Like his younger brother, George was in suit while Charlotte wore a beautiful red and white dress and they eagerly waved to the crowds who had gathered on the Mall, with Louis even tapping his hands along to the music.
Mum Kate shunned military uniform, despite now being colonel of the Irish Guards, and instead wore a green ensemble with a stunning regimental brooch.
It came as King Charles became the first monarch in more than 30 years to take part in Trooping the Colour on horseback in what was his inaugural Birthday Parade.
Charles took the royal salute as Colonel in Chief of the Household Division's seven regiments during the ceremony, staged on Horse Guards Parade, as members of the royal family and 8,000 spectators watch.
The colour, or regimental flag, trooped in front of hundreds of Guardsmen and officers is that of the King's Colour of the 1st Battalion Welsh Guards.
During the Trooping ceremony, the King inspected the Guardsmen, in their scarlet tunics and bearskins, standing in two rows on Horse Guards before the colour was trooped.
The massed bands of the Household Division provided musical backing during the day and also taking part was the King's Troop Royal Horse Artillery who, following the parade, fired a 41-gun salute in Green Park to mark the King’s official birthday – while from the Tower of London the Honourable Artillery Company fired 62 volleys.
Following the ceremony, the royal family will head back to Buckingham Palace and gather on the balcony to watch an extended military flypast after the display on coronation day had to be scaled down due to bad weather.
Around 70 aircraft from the Royal Navy, British Army and Royal Air Force will take part – including aircraft from the Battle of Britain Memorial Flight, the C-130 Hercules on its final ceremonial flight, modern Typhoon fighter jets with a red, white and blue finale from the Red Arrows.
