A touching video has captured Princess Charlotte and Prince Louis waving to their father Prince William ahead of the Trooping the Colour procession.
The siblings were spotted making the sweet gesture to their dad while leaving Buckingham Palace in a horse-drawn carriage they shared with Kate and Camilla.
Prince William can then be seen waving back while on horseback alongside Princess Anne and Prince Charles, resplendent in their military regalia - including their new Platinum Jubilee medals.
Crowds were treated to the sight of George, eight, Charlotte, seven, and four-year-old Louis taking part in their first carriage procession today as historic celebrations marking the Queen's 70-year reign began.
Follow the Queen's Platinum Jubilee celebrations in our live blog here
The nation is embracing the special extended bank holiday weekend of pomp, pageantry and star-studded festivities, which will see celebrities and the public gather in their millions in tribute to the monarch.
Thousands of wellwishers draped in Union flags, party hats and plastic tiaras flocked to central London for the Trooping spectacle.
The young Cambridges and the duchesses' carriage, known as a barouche, was followed by another carrying the Earl and Countess of Wessex and their children Lady Louise Windsor and Viscount Severn.
The Cambridge's children smiled and waved enthusiastically as they sat in a row, with Louis the youngest in the middle of his older siblings. At one point, Charlotte took charge and pushed Louis's hand down as he continued waving.
The princess, dressed in a cornflower blue dress, and her brothers squinted in the morning sunshine during their royal jubilee outing.
The Duke and Duchess of Gloucester and the Princess Royal's husband Vice Admiral Sir Tim Laurence processed in the final carriage.
The Cambridge children bowed their heads when the national anthem was played on their arrival into Horse Guards Parade.
George was dressed in a navy suit, white shirt and blue tie for the occasion, while Louis was wearing a sailor-style white and navy top echoing an outfit worn by his father as a youngster at Trooping in 1985.
Charlotte's hair was worn down, dressed up with two fine plaits fastened with a blue ribbon.
The Queen meanwhile arrived at Buckingham Palace, ready to take a salute from the balcony accompanied by her cousin the Duke of Kent, Colonel of the Scots Guards, when the Household Cavalry Mounted Regiment and the Guards make their way back from the parade ground.
The jubilee monarch, now 96, is facing ongoing mobility issues and opted to stay at her royal residence.
The once-in-a-lifetime jubilee celebrations will see members of the monarchy, celebrities from the UK and across the globe, and people from all parts of the UK gather to recognise the Queen's legacy over the weekend.
The Queen will return to the balcony for a second time on Thursday, accompanied by working royals and the Cambridge children, to watch a fly-past of aircraft by the Royal Air Force.
A service of thanksgiving will be held at St Paul's on Friday, while Epsom Derby Day is on Saturday, and that evening Queen + Adam Lambert and Diana Ross headline the BBC's Party at the Palace.
On the final day, revellers will take to the streets of London for the Jubilee Pageant, that will see each decade of the monarch's reign celebrated with "national treasures" puppets, dancers and music.
