The two young royals, aged one and four respectively, peered out of the windows at Buckingham Palace during the annual parade and waved to the crowds below.
The celebration takes place on the second Saturday every June to mark the Queen's official birthday.
Both Louis and Charlotte were later photographed on the palace balcony watching the traditional fly-past of aircrafts that takes place as part of the ceremony.
The Duke of Cambridge held his 13-month-old son while Charlotte stood alongside Prince George and their mother, the Duchess of Cambridge.
Trooping the Colour 2019: Duchess of Sussex joins celebrations
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Other members of the royal family in attendance on the day included the Duke and Duchess of Sussex, who gave birth to the couple's first son, Archie, on 6 May.
The Trooping the Colour procession, which consists of more than 1,400 soldiers, 200 horses and 400 musicians, moves from Buckingham Palace down The Mall towards the Horse Guard's Parade.
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