The Prince of Wales said he and his wife Catherine have chosen to tell their three children "everything" about the princess's cancer diagnosis, because "hiding stuff from them doesn't work".
Prince William said being honest with Prince George, 12, Princess Charlotte, 10, and Prince Louis, seven, had its downsides and admitted "sometimes you feel you're oversharing with the children".
The 43-year-old made the comments during an interview when he was in Brazil last week for his Earthshot Prize Awards.
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He spoke with broadcaster Luciano Huck while riding a cable car near Sugarloaf mountain.
Huck hosted William's Earthshot Prize on Wednesday last week and shared the interview on Monday to his 23 million Instagram.
Prince William was candid in his discussion about his family's health battles, which saw his wife and father King Charles both diagnosed with cancer in early 2024.
Kate is now in remission after undergoing chemotherapy, which ended in September last year, but the King is continuing to have weekly treatment.
Prince William said: "Every family has its own difficulties and its own challenges and I think it's very individual and sort of moment-dependent as to how you deal with those problems.
"We choose to communicate a lot more with our children. Now that has good things and bad things. Sometimes you feel you're oversharing with the children. You probably shouldn't, but most of the time, hiding stuff from them doesn't work.
"There's no answers, but it's always a balancing act to me that every parent knows that it's kind of: 'How much do I say? What do I say? When do I say?'
"And you know, there's no manual for being a parent. You've just got to go with a bit of instinct."
Prince William spoke about he and Kate's decision not to give their three children mobile phones.
Doing so had "become a little bit of a tense issue".
But he said that when George moves to high school next year, the young prince might be given a "brick" phone with no internet access.
"It's really hard," William said.
"Our children don't have phones. I think when George moves on to secondary school, then maybe he might have a phone that has no internet access.
"And to be honest, it's getting to the point where it's becoming a little bit of a tense issue. But I think he understands why, we communicate why we don't think it's right. And again, I think it's the internet access I have a problem with.
"I think children can access too much stuff they don't need to see online, and so having a phone and text message, the old sort of brick phone as they call them, I think that's fine.
"With full access, children end up seeing things on the internet that they shouldn't. But with restricted access, I think it's good for messaging."
Recently, Prince William told Canadian comedian Eugene Levy that his three children don't have phones.
Soon after, the Princess of Wales said phones were responsible for "an epidemic of distraction" among families, warning of parents being too reliant on their devices.
During their chat, Huck gave William a photograph of Princess Diana taken during her visit to Sao Paulo in 1991.
It showed the late Princess of Wales holding a child with HIV, an image that helped break the taboo surrounding the disease at the time.
Huck said: "She became a very important figure in the fight against prejudice".
William replied: "I carry her social and humanitarian legacy with me every day".
Prince William gave an insight into how he and his wife balance parenting duties with their official engagements.
"I'm the taxi driver," William joked.
"Taxi driver, sports days, matches, playing in the garden, where I can. School run, most days. I mean, Catherine and I share it. She probably does the bulk of it."
The couple frequently schedule their work around the school pick-up and their children's extra-curricular activities, where possible.
The prince spoke about his reasons for being in Brazil, that being the Earthshot Prize which saw five winners awarded £1 million each to develop their ideas to help repair the planet.
"I care a lot about the environment and the world that the next generation are going to inherit, because all the social issues we want to deal with will start from actually our natural world," Prince William said.
"And so if we don't get that right, there's no chance of us being able to feed the world, be able to look after the world, be able to build houses for the world, have space to grow crops, all this sort of stuff.
"It's all intertwined. And so the world I want to pass on to my children is one that I would love to inherit when I was a child. And I think we all want to do that is give, give the world in a better place than when we inherited it."