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.
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.
Camilla, Duchess of Cornwall, Charles' wife, added that the moment was "very touching" and "a lovely gesture". What is going on between Charles and William?
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".