Charles' wife, Camilla, Duchess of Cornwall, discussed Charles' role as a grandparent in the documentary, revealing that the royal is happy to get a little silly with his grandchildren.
Speaking as part of a BBC documentary to mark his 70th birthday, The Prince of Wales told filmmakers that he would rein in his public campaigning when he becomes sovereign.
The heir to the British throne has always been outspoken on issues ranging from climate change and architecture to alternative medicine, and critics have warned that if he continued as sovereign he would put the monarchy at risk.
Harry also noted, "For him, that's a fantastic opportunity to step up and be that support, and you know he's our father so, of course, he's going to be there for us".
In contrast to his mother Queen Elizabeth II, who has remained above the political fray, Charles has tried to intervene in some issues of public concern, writing letters to ministers, speaking out about climate change and opposing an extension to the National Gallery in London.
"I'm not that stupid", he said.
Prince William said "when he's there, he's brilliant" but "we need him there as much as possible".
He said he would "operate within the constitutional parameters" of being the head of the monarchy.
"I will take with me, however, so many special memories of Nigeria, and of Lagos, and of your essential place within our Commonwealth Family".
Hopeful that the upcoming birthday will encourage Charles to slow down, William said "Now that he's reached his 70th year, it's the ideal time to consolidate a little bit".
Their 'children are expected to one day have regular jobs, and will be raised to appreciate the differences in society.
Charles defended his activism as heir to the throne, which includes founding the Prince's Trust charity in 1976 to support vulnerable young people.
William, setting out his own stall, said: "Personally, I want to be my own man, and take my own style, my own passions and my own interests, my way, and do things slightly differently. I don't think people realize quite how kind he is". But the future Duke and Duchess of Sussex were unhappy when told her first choice was impossible, as no one knew exactly where it came from.