The memory of “Granny Diana” is still a big part of daily life in Prince William‘s household.
In a new documentary about Princess Diana, William is opening up how he talks to his children — Prince George, who turned 4 on Saturday, and Princess Charlotte, 2 — about the grandmother that they never got to meet.
In Diana, Our Mother: Her Life and Legacy, which airs on ITV and HBO on Monday, William says he is “constantly talking about Granny Diana.”
“We’ve got more photos up round the house now of her and we talk about her a bit and stuff,” he adds.
“It’s hard because obviously Catherine didn’t know her, so she cannot really provide that — that level of detail,” he says. “So, I do [when] regularly putting George or Charlotte to bed, 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.”
However William jokingly admits that Diana would have perhaps taken to being a grandmother a little too well.
“She’d be a nightmare grandmother, absolute nightmare,” he joked. “She’d love the children to bits, but she’d be an absolute nightmare. 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 opened up about how growing up with a hands-on mother like Diana has shaped his own 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, and having seen, you know, what she did for us.”
Diana, Our Mother: Her Life and Legacy will air Monday at 10 p.m. on HBO in the U.S. and 9 p.m. in the U.K. on ITV.
The show is one of several commemorations of the Princess in print and on film. PEOPLE and ABC have partnered in a two-night television event The Story of Diana, airing on Aug. 9 and 10 at 9 p.m. ET.