'It's Very Special Having a Little Girl:' Princess Kate Opens Up Princess Charlotte's Bond with 'Gan-Gan' the Queen in Extraordinary New Documentary

It's almost Gan-Gan's big 90th birthday.

In a new documentary honoring Queen Elizabeth's milestone birthday on April 21, Princess Kate is opening up about her children's special relationship with the monarch – and reveals a special nickname for her.

Speaking in depth for the first time about the joy of raising her family, including 2-year-old son Prince George and 10-month-old daughter Princess Charlotte, Kate tells ITV's The Queen at Ninety that the royal great-grandmother was "thrilled" when she and Prince Williamwelcomed Charlotte last May.

"It's very special having a new little girl … I feel very, very lucky that George has got a little sister," she says in the new documentary, which airs on ITV in the U.K. on March 27. "The Queen 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."

"George is only 2 ½ and he calls her 'Gan-Gan,' " reveals Kate. "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 appearance marks Kate's first solo TV interview since joining the royal family in 2011. She is among all the senior members of the family, including Prince Charles, William and Prince Harry, who took part in the documentary.

The record-breaking Queen has been a steady right hand for Kate as she learns the royal ropes. Recalling their first joint engagement together in 2012, when she joined the Queen in Leicester, Kate says, "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."

"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."

And Kate adds, "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."

For his part, Prince William talks about how the Queen has enabled him to take his own route. He is the first immediate heir to work in the civilian world. "Growing up, having this figurehead, having this stability above me has been incredible," William says. "I have been able to explore, understand, slightly carve my own path. I greatly appreciate and value that protection."

The former Air Force helicopter pilot underlines the links between his family and the armed forces that the Queen heads. "It is about service to others and helping the community, helping in the national interest."

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"The royal family, and particularly the monarch, has always had a close bond with the armed forces," says William. I think it's to do with shared values, sacrifice, dedication, loyalty, and the Armed Forces don't get enough recognition. So it's a key part of the royal family and the monarch to bring that to light."

Asked what he thinks of how the public sometimes responds to his granny, William replies, "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."

William's brother Prince Harry, who quips that he has been "asking her for years what her secret is, but she won’t tell me,” highlights the impact the Queen has had around the world via the Commonwealth of 53 nations. "I look at the Commonwealth and think, 'Look how much it's achieved,' " he says.

"But what's really encouraging is how much more it can achieve," he continues. "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 colors, different experiences, different islands – whatever it be. Hats off to her. It's incredible."

And he praises the vow she made to serve her country at age 21 after her father's death. "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," says Harry.

The Queen's eldest son and heir, Prince Charles ( the longest serving heir-to-the-throne in history) is looking ahead to the birthday celebrations next month.

"I said to my mother the other day, 'Do you realize that when you reach 90, I shall have known you for 68 years?' She had to laugh a little bit."

"When you think that all these years the Queen has been on throne," says Charles, "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."

Our Queen at Ninety airs March 27 in the U.K. on ITV.