Princess Kate, Prince George, Princess Charlotte and Prince Louis are said to be learning conversational Welsh.
Prince William and Princess Kate are understood to be learning the language with the help of Duolingo and teaching their children.
It follows Prince William's first full message in Welsh to mark St David's Day, demonstrating his growing commitment as the Prince of Wales.
The 42-year-old royal has been learning the language with the help of the Duolingo app.
Princess Kate, Prince George, Princess Charlotte and Prince Louis are said to be learning conversational Welsh
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The Prince's video message was broadcast on Kensington Palace's social media platforms and featured Welsh landscapes and communities.
William wore a daffodil buttonhole and sounded confident in his delivery.
In the video, William said: "Helo. Heddiw, ar Ddydd Gŵyl Dewi, rydym yn dod at ein gilydd i ddathlu Cymru ei hanes, ei diwylliant, a'i phobl anhygoel. O'i golygfeydd anhygoel i'w hiaith, mae Cymru yn parhau i ysbrydoli. Heddiw, rydym yn dathlu popeth sy'n hudol am Gymru. I holl bobl Cymru ac i bawb ledled y byd, Dydd Gŵyl Dewi hapus."
This translates as: "Hello. Today, on St David's Day, we come together to celebrate Wales, its history, its culture and its incredible people."
Prince William and Princess Kate are said to be learning the language with the help of Duolingo and teaching their children
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He continued: "From its breathtaking landscapes to its language, Wales continues to inspire."
"Today, we will celebrate everything that is magical about Wales. To the people of Wales and everybody around the world, Happy St David's Day," the Prince concluded.
A royal source said William "felt it was important" to improve his Welsh.
The Prince revealed last year that he has been learning Welsh "phonetically" rather than taking formal lessons like his father did.
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William and Kate lived on Anglesey, north Wales, for three years from 2010 to 2013
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William and Kate lived on Anglesey, north Wales, for three years from 2010 to 2013 while he worked as an RAF search and rescue helicopter pilot.
The prince has spoken of his "deep affection" for the country.
According to the most recent Annual Population Survey, 851,700 people in Wales can speak Welsh, representing 27.7 per cent of the population of just more than three million.
It is the lowest percentage recorded in eight years despite the language being taught in all schools in Wales up to the age of 16.
