Royal expert Katie Nicholl said Harry and William wore suits to breakfast when they were kids.
She said Charles banned jeans and sneakers when they came to stay with him after his split from Diana.
Diana, meanwhile, took them to McDonald's, the movies, and theme parks.
After their parents' very public separation, Prince William and Prince Harry began splitting their time between the royal estates that King Charles and Princess Diana called home.
And, according to royal reporter Katie Nicholl, Charles and Diana had very different parenting styles.
In her book "The New Royals: Queen Elizabeth's Legacy and the Future of the Crown," which was released in October, Nicholl details what life was like for the two young princes following Charles and Diana's split.
"By the mid-nineties, Charles and Camilla were conducting their love affair behind closed doors, careful not to be photographed together," Nicholl wrote. "The prince divided his time between St. James's Palace and Highgrove, where he could be close to his mistress. Diana was based at Kensington Palace, while William and Harry bounced between all three households and school."
When they stayed with their father, Harry and William "enjoyed country pursuits such as shooting and stalking" and their nanny allowed them to enjoy the "fresh air, an air rifle, and a horse," Nicholl wrote.
But it remained a formal affair. Ken Wharfe, Diana's former protection officer, told Nicholl that he always saw the boys in Turnbull & Asser suits.
"Charles wouldn't allow them to wear jeans and trainers at breakfast," Wharfe said. "He had an almost Victorian attitude to things like that."
Life was very different at Kensington Palace, where Nicholl said the boys enjoyed fast food and fewer rules.
"With their mother, by contrast, they went to the cinema and to restaurants, grabbed the occasional burger at McDonald's, played tennis, and had fun at theme parks," she wrote.
Paul Burrell, Diana's former butler, previously told The Mirror that the Princess of Wales and her two sons would often "nip to McDonald's for a Big Mac and fries before coming back to watch 'Blind Date.'"
Diana also loved taking her boys out for pizza at Da Mario, her favorite pizzeria, which is still open today in London's Kensington neighborhood.
"She used to come in, and it's very well-known that the boys' favorite food was pizza, so they used to sneak in and enjoy our pizzas," Marco Melino, whose father opened Da Mario, told The Society of British & International Interior Design in 2021.
Following Diana's death in 1997, Nicholl said the rules became lax in Charles' household — and the teen boys often acted out.
"With a busy work schedule and Camilla now very much a priority in his private diary, Charles placed too much trust in William and Harry to look after themselves," Nicholl wrote.
William and Harry started "Club H" at Highgrove, where they held parties for their friends, and Harry was drinking alcohol and smoking cigarettes at an inn near the house when he was just 17, she added.
"The boys wanted their independence and they probably had too much of it," an unnamed former aide told Nicholl. "Often when they wanted to speak to their father, he wasn't around… If they needed him they'd end up calling his protection officers so they could talk to him, which wasn't ideal."
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