Charlotte, 16 months, was heard speaking in public for the first time - saying "pop" at the balloons that filled Government House in Victoria.
The residence was transformed into a children's paradise, which included a petting zoo and miniature ponies.
The play day was on day six of the Cambridges' eight-day tour of Canada.
It is the first official trip all four of the family have been on together.
Those who chronicle the lives of the Cambridges now know that balloons are the way to a 16-month-old princess's heart and Charlotte's first public utterance was to use the word "boo".
And for toddler George there was much fun to be had squirting bubbles at his father - even if he is a future king of Canada and the UK.
Considerable column inches are generated from such offerings. The power of the images is considerable.
The children personify the future of the Canadian monarchy in a country that may, one day, chose a different path.
There's no sign, at the moment, of that being the desire of the majority of Canadians.
George and Charlotte, on display, will not become an everyday occurrence.
William and Kate are determined their children grow up in private and not in public. This party will attract global attention because of its rarity value.
Prince William could be heard saying "are we going to go pop?" to his daughter and then asked her: "Would you like a balloon, Charlotte?"
Charlotte is said to made made everyone in the grounds of Government House - the home of the Lieutenant Governor of British Columbia - laugh when she tried to lift up a huge balloon archway that led to the petting zoo.
George appeared excited by the attractions and could not resist squirting Charlotte with it.
The Duke and Duchess of Cambridge were said to be happy that the event had been staged for them and pleased their son and daughter had the opportunity to play with others.
The Duke and Duchess’ royal tour
24 Sept Victoria, British Columbia: The duke and duchess, accompanied by their children, arrived in Victoria, the capital of British Columbia.
25 Sept Vancouver, British Columbia: The duke and duchess visited Sheway, the Immigration Services Society of British Columbia - for an event to celebrate young leaders in Canadian arts, music, sport, charity, business, and film, and then visited the Kitsilano coastguard station.
26 Sept Bella Bella and the Great Bear Rainforest, British Columbia: The duke and duchess travelled to the Great Bear Rainforest, visiting the Heiltsuk First Nations community and attending a reception hosted by the province of British Columbia at Government House.
27 Sept Kelowna, British Columbia and Whitehorse, Yukon: The royals will tour the Okanagan campus of the University of British Columbia and take part in the BC government's "Taste of British Columbia" festival at Mission Hill Winery before flying to Whitehorse, where they will be greeted by members of the Canadian Rangers.
28 Sept Whitehorse and Carcross, Yukon: William and Catherine will visit the MacBride Museum and meet members of Whitehorse's cultural community before travelling to Carcross, where they will be welcomed by the Carcross/Tagish First Nation.
29 Sept Victoria, British Columbia: The royal couple and their children attend a children's party in the grounds of Government House, which will be attended by military families.
30 Sept Haida Gwaii, British Columbia: The duke and duchess visit Haida Gwaii, the archipelago on the northern coast of British Columbia, home to the Haida Nation. They will attend the opening of the new Haida Gwaii hospital and care centre. They will join local youths for a fishing expedition on the waters of Hecate Strait.
30 Sept Victoria, British Columbia: The royal couple will visit the Cridge Centre for the Family, which provides services and support for women who have experienced domestic violence. They will then meet families who have received support from the Kelty Mental Health Resource Centre and later youth working with the Sail and Life Training Society. They end their tour with a public official departure ceremony at Victoria Harbour Airport.