Prince William tells the programme that his father is "brilliant" with his three grandchildren, Prince George, Princess Charlotte and Prince Louis who all feature in an official photo placed prominently behind Charles during one of his interviews in the show. His dad's 70th birthday is November 14.
While Charles said he understood that being king would be different to being heir, he did defend his activism.
"It's something I'm working more heavily on, put it that way".
The duke, 36, was introduced to work with the homeless when his mother smuggled him and brother Harry into a shelter in London when they were boys.
The amusing thing is, the entire time that Prince Charles is pranking his sister, business seems to be going on as usual in the background. "I think people by now would have been rather fed up if I hadn't, I would have thought", he said.
Speaking ahead of his 70th birthday next week, the eldest son of 92-year-old Queen Elizabeth said the role of monarch was completely different from his position as Prince of Wales. He will get down on his knees and crawl about with them for hours, you know, making amusing noises and laughing.
"I don't think we expect the monarchy to be either stony-face silent or to have no views", Lord Hain told the Press Association, adding: "I've never been a monarchist, but if you believe in a monarchy, then I don't see why the monarch now or the future monarch should be expected to be completely view-less and silent on everything".
Just days before their wedding on Saturday, 19 May Harry and Meghan then asked Prince Charles if he would do the honours.
Prince Harry and Meghan Markle are expecting their very first addition to the family, and now, a report claims the Royals are planning to give their offspring a normal upbringing - at least as much as possible!
But dresses by her designer friend Victoria Beckham, who along with husband David Beckham attended Meghan's royal wedding to Prince Harry in May, have made nary an appearance in the Duchess of Sussex's wardrobe rotation - because the 37-year-old doesn't think they flatter her.
He added: "I think it's vital to remember there's only room for one sovereign at a time, not two".