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In a week where she joined Her Majesty and other members of the Royal Family for a special Commonwealth service in the run up to her 90th birthday next month, Kate revealed the sweet name Prince George calls his great-grandmother by.
Prince George calls his great-grandmother the Queen 'Gan-Gan'
"George is only two-and-a-half and he calls her 'Gan-Gan.' She always leaves a little gift or something in their room when we go and stay and that just shows her love for her family."
The Duchess, 34, also shared the Queen's delight at finding out George had been joined by a little sister.
Kate also revealed Her Majesty leaves little gift in their room when George and Charlotte go to Buckingham Palace
As future Queen, Kate also talked about Her Majesty's quiet support for her.
She said: "She's been very generous in not being forceful at all and in any of her views, but I feel she's been there, a gentle guidance really for me."
The Duchess added: "The most memorable engagement for me was an away-day to Leicester. I went without William, so I was rather apprehensive about that. I think there is a real art to walkabouts. Everyone teases me in the family that I spend far too long chatting. So I think I've still got to learn a little bit more and to pick up a few more tips I suppose."
The Queen and Kate during their joint engagement in Leicester in 2012
Recalling how the Queen helped her that day, she said: "She was very supportive. The fact she took the time to make sure that I was happy and looked after for that particular occasion, which probably in everything that she's doing is a very small element, it shows just how caring she is really."
Kate's interview appears in the ITV documentary Our Queen at Ninety, which airs on Easter Sunday.
It also features contributions from the Prince of Wales, Duchess of Cornwall, Duke of Cambridge, Prince Harry, the Duke of York, Princesses Beatrice and Eugenie, the Countess of Wessex and Peter Phillips.
The two-hour programme will show Her Majesty at work and at home and will include tributes from a host of famous faces.
The documentary will show Queen Elizabeth at work and at home
Viewers will also catch a glimpse of the Queen's famous sense of humour as jokes with family members as they prepare to enter a reception for 600 diplomats at Buckingham Palace.
Looking at her watch, she asks: "Is this the right time? Do you think, if we open the door, is there anyone here?"
Her heir, Prince Charles, told the documentary makers : "I said to my mother the other day, do you realise that when you reach 90 I shall have known you for 68 years, she had to laugh a little.
Prince Charles revealed an anecdote that made his mother 'laugh a little'
"When you think that all these years the Queen has been on the throne, that in itself is a huge achievement, coping with so many different challenges and complications and always being there really in that remarkable way – steadfast."
Prince William also paid tribute to his grandmother, saying: "Growing up, having this figurehead, having this stability above me has been incredible. I have been able to explore, understand, slightly carve my own path. I greatly appreciate and value that protection."
Talking about how members of the public react to meeting her, he added: "A lot of people get very excited and sort of nervous around her. And I've seen some very comical moments. I've seen people literally faint in front of her. It's quite a startling moment as to what to do when you faint in front of the Queen. "There's a lot of trembling knees and people can't talk sometimes. It's quite difficult talking to people when they can’t talk. You don't get very far; I don't get past the hellos."
In 2014 a page boy fainted as the Queen delivered her speech during the State Opening of Parliament in the House of Lords
Discussing the UK's world-famous pageantry, the Duke said: "We're very good at pomp and ceremony. In the UK, we're proud of it, but round the world it's seen as a real coup to be able to pull off something like that. For such a small country we pack a big international punch."
His brother Prince Harry also praised his grandmother's devotion to the Commonwealth – whose membership has grown to 53 nations during her reign.
He said: "I look at the Commonwealth and think, 'Look how much it's achieved.' But what's really encouraging is how much more it can achieve. It's a force for good across the whole world, and I think going forward she should be incredibly proud of what she's led and what she’s achieved and what she's created amongst a huge amount of people from different ethnic backgrounds, different skin colours, different experiences, different islands – whatever it be. Hats off to her, it's incredible."
The Queen made her first visit to a Commonwealth country – South Africa – in 1947, where she celebrated her 21st birthday and made her historic broadcast pledging "my whole life, whether it be long or short" to her people.
Princess Elizabeth making a broadcast from the gardens of Government House in Cape Town, South Africa, on the occasion of her 21st birthday
Harry described it as: "a hugely moving speech," adding: "It's just incredible to know that someone at that age appreciated and understood exactly what was expected of them and to basically put duty ahead of everything else so early on in her life is quite remarkable."
Harry also joked: "I have been asking her for years what her secret is, but she won't tell me."
His uncle the Duke of York – the Queen's second son – revealed her extraordinary knowledge of the lives of friends and neighbours.
He said: "Her intelligence network of who's done what, what's happened, who's ill, who's died, who's had a birth, is extraordinary. How she finds out is a mystery."
And Princess Eugenie talked about how "Granny" can truly be herself on her beloved Balmoral Estate in Scotland.
Princess Beatrice and Eugenie with their 'grandpa' Prince Philip
She said: "It's the most beautiful place on Earth. I think Granny is the most happy there, I think she really, really loves the Highlands…walks, picnics, dogs, a lot of dogs, there's always dogs, and people coming in and out all the time. Family-wise we're all there, so it's a lovely base for Granny and Grandpa to be - for us to come and see them up there where you just have room to breathe and run."
Talking about her grandfather, The Duke of Edinburgh, Eugenie added: "I think Grandpa is incredible. He really is strong and consistent. He's been there for all these years, and I think he's the rock, you know, for all of us."