The documentary, called Our Queen At Ninety, which will be aired on ITV during Easter Sunday, offers a rare glimpse into the Queen's private life, as she allowed cameras to follow her for the past year during day-to-day life. The documentary also features Kate and other senior royals paying tribute to her in a series of interviews.
Speaking directly to camera mother-of-two Kate, 34, couldn't help but talk about her little ones as she revealed how 'lucky' she feels having a little girl, in her first solo interview as a royal.
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OK! magazine have reported that Kate starts her interview by saying, 'It's very special having a new little girl, to the family. I feel very, very lucky that George has got a little sister'.
And it looks like Kate isn't the only doting royal over little Charlotte as she reportedly reveals, 'The Queen, she was really thrilled that it was a little girl, and I think as soon as we came back here to Kensington she was one of our first visitors here,' before adding, 'I think she was very fond of Charlotte, always watching what she's up to.'
OK! have also reported that Prince George has a very special name for Her Majesty the Queen, and we don't think it's one she's had before.
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George's mum Kate explains, 'George is only two and a half. He calls her Gan Gan.'
And just like any great-grandmother it sounds like the Queen spoils the two youngest royals as Kate goes on to reveal, 'She always leaves a little gift or something in their room when we go and stay. That just shows, I think, her love for her family.'
In the interview, which is Kate's first ever solo interview as a royal, we also hear her speak about how supportive the monarch has been while Kate has been settling into her role. The Duchess of Cambridge explains,'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.'