She wasn’t just a regular mom — Princess Diana was a cool mom! Prince Harry opened up about his late mother’s fun-filled approach to parenthood in the new HBO documentary, Diana, Our Mother: Her Life and Legacy.

“She was one of the naughtiest parents,” Harry, 32, said in the film. His older brother, Prince William, echoed the handsome redhead’s remark, adding that their mom “was very informal and really enjoyed the laughter and the fun” of raising two boys.

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prince william and prince harry - getty

Prince William (left) and Prince Harry (right).

The documentary coincides with the 20th anniversary of Diana’s death. This is the first time both Harry and William have sat down together to reminisce about their mother, who was killed in a fatal Paris car crash in August 1997. “She understood that there was a real life outside of the palace walls,” said William, who was just 15 years old when Diana passed away.

During a recent (and rare) interview with British GQ, William — who shares son Prince George, 3, and daughter Princess Charlotte, 2, with wife Kate Middelton — said that he often feels sad that Diana never got the chance to meet his family.

“I would like to have had her advice. I would love her to have met Catherine and to have seen the children grow up. It makes me sad that she won’t, that they will never know her,” he explained to the magazine. “I am in a better place about [my mom’s passing] than I have been for a long time, where I can talk about her more openly, talk about her more honestly, and I can remember her better, and publicly talk about her better.”

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The father-of-two added, “It has taken me almost 20 years to get to that stage. I still find it difficult now because at the time it was so raw. And also it is not like most people’s grief, because everyone else knows about it, everyone knows the story, everyone knows her.”