May 14, 2015 02:55 PM

Prince George‘s appearance on those famous hospital steps might have lasted less than a minute, but thanks to the “George Effect,” his toddler style is already making a big impact.

The cardigan he wore to meet his new sister, Princess Charlotte, “was not even available, as it’s winter stock, but we had a similar one called the Sous Marin and that sold out in every color,” Amaia Arrieta, the owner of Amaia boutique in London, tells PEOPLE. “We sold out in less than 24 hours!”

Luckily for George’s ever-growing fan base, the exact cardigan Kate bought back in November is due back in stock any day.

“It’s on its way! There is a waiting list, so once we have fulfilled those orders, the cardigan will back in stock on our website,” says Arrieta, who owns the tiny, fashionable children’s wear boutique in Chelsea Green with her partner, Segolene Tresca.

Anwar Hussein/WireImage

“It’s a very particular shade of blue called ‘night blue,’ so it took a while to dye the knit at our factory in Spain,” she adds. “But everyone has been working overtime to rush the order through.”

Kate is such a fan of the style that she snapped up several of the more lightweight Sous Marin cardigans just a few weeks ago.

“She likes the round-neck styles because she likes when it goes all the way up to the neck,” says Arrieta, adding: “She bought three of those recently.”

Fans of those adorable long socks can also rejoice. The stretch-ribbed knit socks had sold out in several colors, but they are quicker to manufacture than the cardigan and are now back in stock in all 15 colors

“Everyone loves the socks — we’ve had orders from all over the world — including Australia — which means the postage will cost more than the socks, which is crazy!” says Arrieta, who has run the boutique for nearly a decade.

So why is the world so fascinated with a 21-month-old’s style?

“I think it’s because he is so cute and he makes everything look adorable, but also because it’s a traditional look that people will never get bored of,” says Arrieta. “That’s the appeal.”

— Monique Jessen

For much more on life with Prince George and Princess Charlotte, pick up this week’s issue of PEOPLE, on newsstands Friday

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