Prince William wishes his workaholic father could spend more time with his grandchildren, the documentary to mark Charles' 70th birthday reveals.
The Prince of Wales is described as a "brilliant" grandfather who will spend hours crawling on the floor and making silly noises with his little granddaughters and grandsons.
But the prince can be distracted because he is so dedicated to his work – to the extent that his sons say he sometimes falls asleep with the documents he is reading stuck to his face, reports Daily Mail.
Asked whether his father has "time to be a grandfather on top of everything else", William answers carefully.
In recent years, Charles is said to have privately expressed his frustration that he doesn't get to see as much of his grandchildren as the Middleton family, a claim his aides have always furiously dismissed.
But his elder son makes clear that he would like to have more "family time" with his hard-working father.
William says: "It's something I'm working more heavily on, put it that way. I think he does have time for it, but I would like him to have more time with the children.
"Now he's reached his 70th year it's a perfect time to consolidate a little bit because, as most families would do, you are worried about having them around and making sure their health's OK – and he's the fittest man I know but equally I want him to be fit until he's 95.
"So having more time with him at home would be lovely, and being able to play around with the grandchildren.
"Because when he's there, he's brilliant. But we need him there as much as possible."
Camilla adds: "He will get down on his knees and crawl about with them for hours, you know making funny noises and laughing, and my grandchildren adore him, absolutely adore him. He reads Harry Potter and he can do all the different voices and I think children really appreciate that." Aides say Charles could not be prouder to be a grandfather.
The prince happily shows the documentary crew an arboretum that he has planted for his eldest grandchild at Birkhall, his Scottish home, which he calls "George's wood".
Referring to his passion for the environment and life-long campaign to highlight the perils of climate change, Harry reveals his father still can't help "banging the drum" even when he sits down with his sons to dinner.
He tells his brother: "You know how frustrated he gets. But he's done an amazing job, and without telling us what he should be doing or the direction that we should go in, he's just let us learn from the nature of the job, learning from him, learning from Mummy."
The programme reveals how Charles would take his sons litter picking when they were on holiday with him.
He has also passed down some of his other quirkier traits.
Harry reveals: "He's a stickler for turning lights off.
"And that's now something that I'm obsessed with as well, which is insane because actually my wife certainly goes 'Well why turn the lights off? You know it's dark'.
"I go 'We only need one light, we don't need like six', and all of a sudden it becomes a habit and those small habit changes he's making, every single person can do. And I think that is one of the key lessons certainly that I felt that he taught us."
William agrees, adding: "I know I've got serious OCD on light switches now which is terrible.
"He does life the way that he advocates. He did take to heart the criticism quite a lot when he was younger."
If there is one thing the prince's family is agreed on, it's their desire – not that he will ever listen – for him to slow down.
Harry jokes: "He does need to slow down, this is a man who has dinner ridiculously late at night.
"And then goes to his desk later that night and will fall asleep on his notes to the point of where he'll wake up with a piece of paper stuck to his face."
William agrees, saying: "He has amazing personal discipline. So, he has – and it's frustrated me in the past a lot – he has a routine.
"The only way to fit all this stuff in is things have to be compartmentalised. The man never stops.
"I mean when we were kids there was bags and bags and bags of work that the office just sent to him. We could barely even get to his desk to say goodnight to him."
The Duchess of Cornwall says she tried to encourage her husband to "pace himself" but admits he is too driven to take any notice.
She smiles ruefully: "My father once said to me, 'As you get older, you've got to do one thing and that is pace yourself'. And I would love to tell him to pace himself, but I'm afraid that's not going to happen."
MAN OF DESTINY
Charles neatly sums up the way his life has been mapped out before him, with astonishing matter of factness.
He says: "When my mama succeeded and became Queen when by grandfather died so young, aged 57, I ended up becoming heir to the throne at aged four."
And yet his wife says it is a weight he bears lightly.
Asked whether the crown is something he often thinks about, Camilla says: "No, I don't.
"I think his destiny will come, he's always known it's going to come and I don't think it does weigh on his shoulders at all."
When she is asked if it is something he talks about, the duchess replies: "Not very much, no. It's just something that's going to happen."
Asked to give a message to the prince on his birthday, his sons are quick to reply.
Harry laughs: "Please have dinner earlier is my message to him! I would encourage him to remain optimistic because I think it can be very easy to become despondent and negative.
"But with hopefully his children and his grandchildren and hopefully more grandchildren to come, he can get energy from the family side and then carry on his leadership role."
William adds: "I'm very pleased that he's made 70. You know if you ask him if the job's done, it's not.
"But more than anything I'd like to see his passions and his interests and the things he's been campaigning for come to fruition completely for him. He hasn't even reached the point that his natural progression should do – i.e. being monarch. So, you know he's still got his job to do."
Prince, Son and Heir - Charles at 70 is on BBC One at 9pm tonight.