Princess Diana didn't live to meet her grandchildren, Prince George, 4, and Princess Charlotte, 2.
But that hasn't stopped her eldest son, Prince William, 35, from imagining what life would be like if she hadn't died at 36 — just one year older than he is now — after a tragic car crash in Paris nearly 20 years ago.
In a new special on the late princess, ITV and HBO's "Diana, Our Mother: Her Life and Legacy," which airs on July 24, William opens up about his mother, his kids with Duchess Kate, and how he's teaching the youngest royals about the grandmother they'll never know.
"[I'm] constantly talking about Granny Diana," William says in the special, reports People magazine.
"We've got more photos up round the house now of her and we talk about her a bit and stuff," he reveals. "It's hard because obviously Catherine didn't know her, so she cannot really provide that, that level of detail."
Sometimes, when William puts George and Charlotte to bed, he brings up his mom. "[I] talk about her and just try and remind them that there are two grandmothers, there were two grandmothers in their lives, and so it's important that they know who she was and that she existed," he says.
If Diana was alive now, "She'd be a nightmare grandmother, absolute nightmare," William quipped.
"She'd love the children to bits, but she'd be an absolute nightmare," he says, explaining, "She'd come and go and she'd come in probably at bath time, cause an amazing amount of scene, bubbles everywhere, bathwater all over the place and — and then leave."
William also reveals that unlike many older generations of royals, he strives to be a hands-on dad, something inspired by Diana's loving parenting style.
"I want to make as much time and effort with Charlotte and George as I can because I realize that these early years, particularly, are crucial for children," he says, "and having seen, you know, what [my mother] did for us."
In the TV special, Prince Harry, 32, also makes a confession about his mom: She might have had something to do with his well-earned reputation as the most mischievous bad boy of the royal family — one that eventually earned him the nickname "the Party Prince."
"She was one of the naughtiest parents… One of her mottos to me was, 'You can be as naughty as you want — just don't get caught,'" Harry admits, as reported by the Mail on Sunday. "[She was] a total kid, through and through."
"She would come and watch us play football and smuggle sweets into our socks," Harry recalls, adding that he once had to find a way to hide five packets of Starburst candies she'd given him. "The whole shirt was just bulging with sweets, and then look around, open the tuck box, throw it all in, lock it up… Behind closed doors, she was a very loving caring mother — and an incredibly funny person."