Now, of course, the couple are proud parents to three children, and William spoke lovingly about his young family while speaking to a local child and her mother in A Planet For Us All.
WATCH: The Cambridge children have some questions for Sir David Attenborough
Shaking the little girl's hand, 38-year-old William could be seen laughing and joking as he asked her: "Do you like trouble? You are like my little Charlotte! How old is she?" "She is three," replied her mother.
"She is three years, exactly like my Charlotte!" the Prince exclaimed.
Prince William has said fatherhood has changed his outlook
He explained: "You can be a happy-go-lucky, young guy, enjoy parties, and then all of a sudden you go, 'There's a little person here, and I am responsible for that person.'
"Now I've got George, Charlotte and Louis – they're my life, and your outlook does change."
William and Kate are very proud parents to Prince George, Princess Charlotte and Prince Louis
Later in the documentary, William shared further surprising details about his family when he paid a visit to Liverpool to surprise a group of children who had built a bug hotel, named Bugingham Palace.
"Is Princess Charlotte cheekier than Prince George?" one little girl called Daisy asked the Prince.
"No they're about as cheeky as each other," William replied. "They're very cheeky."
Another girl, Poppy, then asked: "Has Prince George learnt (sic) you the floss?"
George and Charlotte are 'as cheeky as each other', according to their dad
"No, Charlotte can floss," says William. "She can already floss at four. Yeah, you don't want to see me floss. Catherine can floss but I can't. It's, it's like a really horrible film to watch me floss."
Also in the new documentary, the Duke reflected on how his children have impacted his commitment to protecting the environment.
"I really want to make sure that in 20 years, George doesn't turn around and say, are you ahead of your time? Because if he does, we're too late," William said.
"Now I've got George, Charlotte and now Louis in my life, your outlook does change. You want to hand over to the next generation, the wildlife, in a much better condition."
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