Best photos of Kate, William, George and Charlotte at King's Cup regatta
Prince George and Princess Charlotte, wearing nautical-themed blue outfits, were looked after by maternal grandparents Michael and Carole Middleton for the day, spending the morning at a local fair to take part in a tug'o'war, balloon and bubble making, and learning how to be a pirate.
The duchess's boat and several others were disqualified in the second race.
Speaking to ITV News after meeting the royal, the writer revealed: "He said it's his daughter's birthday today and he loves the book".
How adorable did Princess Charlotte and Prince George look as they joined their parents at the King's Cup sailing regatta on the Isle of Wight on Thursday?
George's sister Princess Charlotte, 4, watched as well, goofing off with her mom after the races concluded.
Although this is the inaugural King's Cup Regatta, which benefits several of the Duke and Duchess's patronages, the physical trophy given to the victor has a long association with the British royal family.
The mother-of-three finished in last place and was hilariously awarded a giant wooden spoon for her efforts.
The competition was for a good cause, benefiting charities for which William and Kate are patrons.
The race is a charity event, which sees eight boats compete to win The King's Cup. Kate competed on behalf of Early Years, a program Kate started with The Royal Foundation to help give young children their best start in life.
"It's the taking part that counts", the official Kensington Royal Instagram page joked, alongside side two photographs of an embarrassed Catherine being awarded her spoon.
The Cambridges congratulated winning skipper Bear Grylls as he was presented with the King's Cup trophy.
The first ever King's Cup saw the Royals - along with celebrities - enjoy the Solent in ideal conditions.
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.