Comedian John Bishop and adventurer Bear Grylls also competed.
Kate arrived at the event wearing a nautical-themed outfit - a striped Sandro top, LK Bennett pants and her Superga trainers - before changing into her sailing gear.
See the best photos from the King's Cup regatta in our gallery.
At Meghan Markle and uncle Prince Harry's May 2018 wedding, Princess Charlotte had stuck her tongue out back then as well as their auto arrived at St. George's Chapel in Windsor.
William's team came in fifth during the second race, but Kate's team was disqualified for starting too early.
Princess Charlotte drew laughter from the crowd when she stuck her tongue out at them, instead of waving.
Princess Charlotte has had her fair share of cheeky moments.
The event was due to take place tomorrow (Friday) but anticipated weather forced organisers to push things forward by 24 hours.
The race was held a day early after the Met Office warned of rain, high, winds and thunder.
Kate and William are strong advocates for sports, and have undertaken work both in the United Kingdom and overseas to highlight the positive impact that athletics can have on young people.
The message said: "The eight boats taking part in The King's Cup will each represent one of the charities that are being supported this year by Their Royal Highnesses".
The duke trounced his wife when they competed in a dragon boat race in Canada in 2011, but Kate got her revenge three years later when she won a sailing event in Auckland during their tour of New Zealand. Cute! As ever, all eyes were on the sweet outfits that the Duchess of Cambridge chose to dress her children in for the occasion - and of course, the sailing event called for plenty of nautical stripes.
William was competed for Child Bereavement UK, while Kate opted to support four charities on behalf of the Royal Foundation. Her older brother, Prince George, who just turned 6, was close by, rocking a black-and-white striped shirt. Spectators watched the race from a 800-seater grandstand.
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