Sienna, the granddaughter of the Duchess of York, was treated to a special unicorn-themed party to celebrate her third birthday.
Sarah Ferguson said how her eldest daughter Princess Beatrice turned to Tesco for Sienna's cake.
The 64-year-old, who lives in Royal Lodge in Windsor, was speaking to Hello! magazine about how Beatrice and her husband Edoardo Mapelli Mozzi also hosted Princess Eugenie, Jack Brooksbank and their two sons August and Ernest.
She also revealed that her daughter was especially happy with the present she bought Sienna.
Edoardo Mapelli Mozzi walking with his children, Wolfie, eight, and Sienna, threeHRH Princess Beatrice/Buckingham Palace
Princess Beatrice's daughter Sienna was treated to the special day
PA
Sarah Ferguson said: "We had such a good time, with balloons and a fabulous unicorn cake from Tesco...I also gave her a pink mackintosh with glitter in it and I think mum was thrilled with that!"
Sienna Elizabeth Mapelli Mozzi was born at Chelsea and Westminster Hospital in London on September 18 2021, with her middle name a nod to her late great-grandmother Queen Elizabeth II. Sienna has an older half-brother, Christopher Woolf, known as Wolfie, from her father Edoardo's previous relationship with American architect, Dara Huang.
Beatrice, who is Vice President of Partnerships and Strategy at Afiniti, and Edo are raising their daughter to be a private citizen and have never shared a photo of Sienna's face.
Earlier this year, one of Beatrice's friends told Tatler: "Sienna is absolutely gorgeous and looks exactly like Bea. She's really cheeky and has a very good personality...Bea has such a good heart and she's such a kind person, so motherhood came very naturally to her; she's a fantastic mum."
LATEST ROYAL NEWS:
Sienna Mapelli Mozzi standing next to her son August BrooksbankInstagram / Princess EugenieFergie has stressed the importance of teaching children the importance of the natural world as well as sharing her love of reading.
The Duchess also aims to get youngsters to "stop swiping" on smartphones in her upcoming new book.
She told Hello!: "I was brought up on a farm where my father taught me to believe in the power of nature. I want my grandchildren to see a harvest with poppy fields.
"I read my books to them all the time. They love it; it's one of my favourite things to do with them. I think I watch them and the ideas [for the stories] come from that."
Sabrina Elba, Sarah Ferguson, Duchess of York, and Brianna Fruean speak during the Climate Solidarity in the Commonwealth panel at Global Citizen NOW
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Sarah and her daughters are all passionate about saving the planet, with the Duchess joining forces with Sabrina Elba in New York to lead a discussion for Global Citizen NOW's Health & Climate Financing Sessions.
She was supported by her daughter Princess Beatrice, for the official launch of her Youth Impact Council organisation.
