The author's eldest daughter turned 35 on 8 August and on the latest episode of her podcast, Tea Talks with The Duchess and Sarah, she recalled the day with Beatrice and her one-year-old daughter Sienna.
Proud grandmother Sarah said: "I hid many different fairies in the garden, and I sent Sienna on a fairytale hunt to find the fairies, with wings on. She lasted with the wings on for quite a while before Beatrice and I put the wings on. And then we had a fairy picnic once she'd found the fairies, and that was a magical day for Beatrice's birthday."
The Duchess, who has penned several children's books, added: "I seriously was making Sienna understand there were fairies that are living in the oak tree in the garden so she had to go and find the little door to the fairy department store, which is the oak tree. Of course that's my next book I've just started writing about the department store with the Sluggington Smyths, which are a slug family, a very fine slug family."
Ahead of Beatrice's special day, Sarah revealed on a previous episode of her podcast that she had been on babysitting duties with little Sienna while Beatrice and her husband, Edoardo Mapelli Mozzi, went away.
The Duchess shared how she and her granddaughter had been preparing for Beatrice's birthday: "We're busy blowing up balloons and she's made loads of cupcakes. Well, she told GG how to make the cupcakes and then GG and Sienna had to go and try the cupcakes and then we had to put our fingers in the cupcakes, so we've been very adventurous in the kitchen, it has to be said."
And in another sweet anecdote, Sarah spoke about how being a grandmother had taken back to her early days of parenting.
The Duchess, who is known as GG to her grandchildren, added: "She [Sienna] does the same thing as Beatrice. Beatrice is a Leo and it's taken me right the way back to following Beatrice around, literally mini-me. Off she goes, hunting for ducks and walking through puddles, the rain on her head, and then sings 'Raindrops keep falling on my head…plop'. Well, I sing the first bit and then she goes 'Plop'. So we've had an extraordinary time, I'm very glad Beatrice and Edo got some time alone too."
Sarah also revealed Sienna's adorable nickname as she appeared at The 92nd Street Y in New York back in March during her book tour. Find out in the clip below…
Sarah is also a grandmother to Princess Eugenie's two sons, August, two, and baby Ernest, who was born in May.