It has not been confirmed who will join Her Majesty on the iconic viewpoint, but royal watchers are hoping Kate will make a second appearance.
The little princess wore a head-to-toe blue ensemble with a printed puff-sleeve dress and Mary Janes, while her brother opted for a dapper button-down, dark shorts and classic black loafers.
According to Royal biographer Ingrid Stewart, despite its grand surroundings, the intimate tone of the event was set by none other than the Duchess of Cambridge.
More closely, Kate is a former RAF wife herself, with William having earned his wings - along with Prince Charles, Prince Philip and King George VI - in 2008.
Anyone meeting Meghan Markle for the first time might easily be star-struck - a well-known actress turned duchess isn't someone you come across very often. She paired the look with nude heels and a clutch.
Prince Charles, Camilla, Duchess of Cornwall, and Queen Elizabeth II also attended the event at Westminster.
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And he's not the only child who had a cute reaction to meeting a member of the royal family.
The dress appeared to be the exact same style to the lemon one she wore to Prince Harry and Meghan Markle's May wedding, which is also identical to the cream one the Duchess wore for Princess Charlotte's Christening in 2015.
The newlyweds attended the church service followed by the parade and fly over, before taking off for Ireland later in the day, for their first foreign tour since their wedding.
The Duchess of Cambridge is thought to have given a special nod to her late mother-in-law on Monday (July 9).
Kate last wore the broach - once the highest award any woman of the rank could achieve - during a service for the 75th anniversary of the RAF Air Cadets in 2016.