The Princess of Wales spoke about her youngest son Prince Louis' reaction to the Queen's death on Saturday, whilst taking part in a walkabout outside Windsor Palace.
The moment was revealed thanks to a royal reporter on the ground, who spoke to wellwisher Banita Ranow, 28, who had listened in on the conversation.
WATCH: Kate opens up about Prince Louis' reaction to Queen's death
A video, however, has now been released on TikTok, showing the Princess' exact words.
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In the brief clip, Kate can be heard saying: "My little Louis is just so sweet. He said, 'Mummy don't worry, because she is now with great grandpa."
The Princess of Wales spoke to hundreds of wellwishers outside Windsor Castle
Towards the end of the video, the mother-of-three can be seen holding back the tears.
Prince William and Kate's appearance was incredibly moving to many as it saw them reunited with the Duke and Duchess of Sussex for the first time in over two years.
The outing came just minutes after Prince William released an emotional statement in which he paid tribute to his "Grannie".
It began: "On Thursday, the world lost an extraordinary leader, whose commitment to the country, the Realms and the Commonwealth was absolute. So much will be said in the days ahead about the meaning of her historic reign.
Thousands of royal fans took to Windsor over the weekend
"I, however, have lost a grandmother. And while I will grieve her loss, I also feel incredibly grateful. I have had the benefit of The Queen's wisdom and reassurance into my fifth decade. My wife has had twenty years of her guidance and support. My three children have got to spend holidays with her and create memories that will last their whole lives.
"She was by my side at my happiest moments. And she was by my side during the saddest days of my life. I knew this day would come, but it will be some time before the reality of life without Grannie will truly feel real."
He continued: "I thank her for the kindness she showed my family and me. And I thank her on behalf of my generation for providing an example of service and dignity in public life that was from a different age, but always relevant to us all.
"My grandmother famously said that grief was the price we pay for love. All of the sadness we will feel in the coming weeks will be testament to the love we felt for our extraordinary Queen. I will honour her memory by supporting my father, The King, in every way I can."