Princess Charlotte appeared in a floral print dress with puff sleeves by children's designer Rachel Riley — and the outfit quickly sold out
Princess Charlotte
Princess Charlotte
| Credit: Duchess of Cambridge/PA

At just 6, Princess Charlotte is already becoming a style queen!

Last weekend, Kate Middleton, 39, and Prince William, 38 shared a new portrait of their daughter to celebrate her sixth birthday. Princess Charlotte appeared in a floral print dress with puff sleeves by children's designer Rachel Riley — and the $82 outfit quickly sold out.

"It was such a treat to see! It was a delightful surprise, I wasn't expecting it at all and was gardening at the time!" Riley told Hello! "I became aware of the picture when my friends and family started sending me messages asking if the dress was one of ours!"

The designer added that the dress completely sold out within 12 hours, making it the fastest seller in the brand's history. They even set up a preorder system for customers eager to replicate Charlotte's sunny style.

Cambridges attend pantomime
Prince Louis, Kate Middleton and Princess Charlotte
| Credit: AARON CHOWN/POOL/AFP via Getty

Princess Charlotte's mom is known for "The Kate Effect," with items she wears quickly selling out — and Riley sees that Charlotte is already following in those footsteps.

Get the premiere issue of PEOPLE Royals for glamorous new photos and inside stories royals fans haven't seen or read elsewhere! Subscribe at

"Personally, I think the Duchess of Cambridge always gets it just right when it comes to her fashion choices. She combines high street with designer, wears lots of British brands and is known for wearing outfits on several occasions," the designer told Hello! "I have no doubt that Princess Charlotte will become as much of a style icon as her mother is, and I look forward to seeing Princess Charlotte's style develop as she continues to grow and mature."

Catherine, Duchess of Cambridge and Princess Charlotte of Cambridge
Princess Charlotte and Kate Middleton
| Credit: Pool/Samir Hussein/WireImage

While Princess Charlotte is taking after her mom's fashion influence, her new birthday portrait quickly drew similarities to Prince William. Photos of Prince William around the same age show off their similarities, from that gleam in their eyes to their cheeky grins.

Prince William, Princess Charlotte
Prince William; Princess Charlotte
| Credit: Tim Graham Photo Library via Getty; Duchess of Cambridge via Getty

Can't get enough of PEOPLE's Royals coverage? Sign up for our free Royals newsletter to get the latest updates on Kate Middleton, Meghan Markle and more! 

On Tuesday, William visited Babcock Vehicle Engineering to learn about the company's important role during the COVID-19 pandemic, when they created a new prototype of ventilators. The company's lead HR business partner Jenna Jackson asked the royal dad about Princess Charlotte's birthday on Sunday.

"She had a lovely day, thank you," said William, according to Hello! "Last year it was her birthday in lockdown, but this year we were able to have one other family over. They grow up very fast. It was great fun."