The ‘Kate Effect’—i.e. whatever Kate Middleton wears sells out instantly—is such a well-known fashion phenomenon that cynical profiteers have taken to snapping up her latest coat dress or hat on Amazon, and reselling the items on eBay before they have even been delivered.

And increasingly it seems that Prince George and Princess Charlotte’s outfits may be having a not dissimilar effect on the kids’ clothes that Kate dresses them in.

Indeed, the 2015 John Lewis Retail Report, noted that “Prince George’s traditional outfits lead to increased sales of boy’s navy knitted jumpers by 69% and red cords by 60%.”

When George wore a quilted jacket for his first day at school, the retailer experienced a 447 per cent increase in enquiries for similar items.

Kate’s tastes are certainly traditional, and these clearly extend to her kids. George is rarely seen in public without knee-high socks.

Last weekend, attending the Trooping of the Color, George wore, as noted by the website What Kate’s Kids Wore, a near identical outfit to that worn by his father Prince William in 1986—blue shorts and a simple blouse with a blue-piped petal collar.

Susan E. Kelley, the editor of What Kate Wore and its sister site, the practically named what Kate’s Kid’s Wore, told the Daily Beast that following public appearances such as last weekend’s, “I see an immediate increase in email from readers asking about whatever style George is wearing: where to purchase it; how to go about ordering from EU countries; questions about the possibility of it being reissued by a retailer.”

The reason behind the interest in George’s clothing is pretty simple, she says.

“More than anything, I think mothers, grandmothers and other shoppers like the cachet of putting their infant or toddler in the very same piece Prince George has worn.”

So simple snobbery then?

Not entirely. “We have been experiencing something of a renaissance in classic styles for children: pieces with timeless lines and handworked details like smocking are increasingly desirable,” she says.

While George wears his fair share of Bonpoint and Petit Bateau, smaller brands—Rachel Riley, Trotters, Amaia Kids and Irulea00have received amazing exposure due to the young prince’s patronage.

Kelley says: “A good example of a small company being able to benefit from the Prince George effect involves Spanish manufacturer Fia Enrique. They made the blue jumper George is seen wearing in the Cambridge Christmas card photo, a piece that sparked a frenzy of interest. People loved the Fair Isle design and the price point was also attractive: about $32 for US buyers.”

Understandably, few brands that dress the royals will speak publicly about their relationship with the household for fear of killing the golden goose. If there is one thing Kate hates, it a blabbermouth.

But one woman who knows Kate’s style well is Cecile Reinaud, the founder and head designer of Séraphine, the international maternity fashion label and store which become one of Kate’s go-to brands when she was pregnant.

Reinaud is freer to talk than most having recently been commissioned by the Diana Award foundation to create a signature baby wear range, with twelve percent of the sales of every piece in the range going towards the foundation to raise funds for their mentoring and anti-bullying programs.

Reinaud told the Daily Beast: “The Duchess of Cambridge has a very good eye for fashion and is an expert at wearing classic styles in a very modern way. She especially likes simple, streamlined silhouettes and I think that has had a great influence on fashion designers and crucially on women who want to emulate her style.

“For children’s wear she is definitely into retro looking outfits which is adorable for toddlers and is becoming a real fashion trend thanks to her.

“Kate likes to mix styles on George, so for example she will put him in a classic pair of boys shorts but with a cute and fun jumper like the one with the little soldiers from Cath Kidston. She’ll team a classic cashmere cardigan with a pair of crocs shoes to give a contemporary twist.

“Prince George is such a little character with bundles of personality, and I love how his style reflects that. There are plenty of influences from classic and retro children’s wear shapes, but they’re pared down to suit a more modern aesthetic.”

Kate’s style may not be to everyone’s taste, but her influence over fashion sales is beyond doubt.

The Kate Effect has been estimated to be worth over $1bn. A recent example came when she wore a printed dress by Anita Dongre to play cricket in Mumbai, and the designer’s factory had to temporarily cancel the production of all other lines.

“The factory is just producing the dress now. Everyone is focusing on that. It is all we are producing now,” Dongre told the Wall Street Journal at the time.

And if Prince George is not causing quite the same retail panic at the tills just yet, well, men’s fashion has never been quite such a fast and furious affair as women’s.

You want to see kid’s clothing sales juiced up by a royal endorsement? Just wait till his little sister wears her first party dress.