She was offered a bouquet by the two-year-old daughter of British expat David Cheater.
Kate smiled at the youngster named Raffaela and laid a hand on her head, telling her father: “What a beautiful daughter you have”.
And when David wished the duchess luck in her own efforts to start a family, she thanked him, adding: “Yes, I hope to”.
She was clad in her first outfit of the day, a nude-hued Vanessa dress by Joseph paired with her trusty nude platforms.
It is the first time Kate has spoken publicly about having children, although shortly after Prince William proposed, he revealed that they plan on starting a family.
The walkabout came on day four of their official North American tour, after they donned chef’s whites for a cookery lesson in Montreal.
And the pair later attended a Freedom of the City Ceremony outside City Hall.
William was met with warm applause when he gave a short speech in French, thanking his audience for their “patience" with his accent, and promising to return.
The next stop was a project for young homeless people.
Kate had made a quick outfit change into an Erdem number. The electric blue Jacquenta dress featured floral lace applique across the shoulders and sleeves and a split skirt.
She accessorised with nude patent platform shoes and a matching mock-croc clutch bag.
It was the second time she'd worn a design by the Canadian-born British creator. On arrival in Ottawa last week, the Duchess was in a lacy number of his that also won plaudits.
At the drop in centre the couple managed to win themselves another fan - a punk with blue and green hair named Pierre.
A circus skills student, he was thrilled when the Duke described his juggling as “awesome”.
The 24-year-old, who was living on the streets before the centre helped him find a home, was eager to praise the newlyweds, saying it was “special to meet them, a privilege”.
They were shown around the centre by Steven, 21, who wore a bullet through his earlobe.
Competitive Prince William took Steven on in a game of table football, shouting “Come on the claret and blue”, in a reference to Aston Villa, the football team he supports.
More on William and Kate's debut tour