While Kate waved at onlookers in Cowes, Charlotte showed her cheeky side by poking her tongue out as she peered out of the window.
She and Prince William, 37, competed against each other and a host of other celebrities at the charity race, eventually losing to Bear Grylls.
The event was held off the Isle of Wight and was brought forward a day due to the weather forecast.
Prince William finished third while Kate finished last and was handed a wooden spoon.
After lifting the trophy, Grylls appeared to suggest that George had eaten an ant when he said: "And also Prince George, your first ant you ate today".
The pictures showed that it was a fun day for the Royal family at the King's Cup regatta.
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Ahead of the first race, the boats were recalled to the starting line after an initial false start in which some of them were over the line.
The boat, along with six others, crossed the starting line too early and was supposed to go back, but instead carried on with the race regardless.
In 2017, William, Kate and Prince Harry participated in a training session for runners taking part in the London Marathon. Despite their royal status, neither William nor Kate took the top prize.
Kate and William were seen posing alongside the King's Cup charity ambassadors, Fara Williams, Dan Snow, Bear Grylls, John Bishop, Katie Thistleton, and Helen Glover before boarding their boats.
The royals are hosting the inaugural regatta The King's Cup to raise awareness and funds for eight of Their Royal Highnesses' patronages.
Kate and Prince William both competed separately in the yacht race, with Charlotte and her older brother Prince George watching on from another boat with Kate's parents, Michael, 70, and Carole Middleton, 64.