The premium brands that regularly feature in the wardrobes of pint-size A-list offspring, and the retailers that sell them, are reaping the rewards. Net-a-Porter has registered the domain name Petite-a-Porter, and Harrods of London recently expanded its children’s wear department to 66,000 square feet. When a figure like Harper is spotted wearing one outfit, brands say, sales can skyrocket and pieces sell out only hours after a photograph appears online.

“Our loyal customers are always interested in seeing who is wearing our clothes, and we do find that the clothes worn by Harper sell very quickly,” said Princess Marie-Chantal of Greece, creative director of the luxury children’s label Marie-Chantal in London. Pink velvet party dresses from the brand sell for around $280, while a pillar-box red coat with a bow collar costs $395.

“Victoria has been a great supporter over the years, and Harper has been wearing Marie-Chantal since she was a baby,” the princess said. “We are so happy that Harper still loves wearing our clothes even as she gets older.” (The Harper Beckham blog lists 31 times that Harper has been photographed in Marie-Chantal.)

Eleanor Robinson, head of accessories and children’s wear at Selfridges, said last week that best-selling designer items in recent seasons for the upmarket British department store have included a Burberry trench worn by Romeo Beckham, Pink Dr. Martens and Charlotte Olympia kitty flats seen on North West; and items by Chloé, Bonpoint and Little Marc Jacobs that Harper has been spotted wearing.

“Here in the United Kingdom especially, there is this very British sense of pride with celebrities and their children, and in particular with the Beckhams and Princess Catherine and Prince William,” Ms. Robinson said. “These are family role models who dress their children beautifully, and consumers aspire to replicate this personal sense of style that also reinforces positive family ideals. Children’s wear has become an increasingly important part of our fashion business.”

The person who runs the Harper Beckham fashion site, Anastasia Medvedeva, is a blogger based in the San Francisco Bay Area. She said the site got about 3,000 hits a day and was an “unremarkable” moneymaker, deriving its income from a few Google ads. “I first started eight years ago with a fashion blog about Suri Cruise because I loved her style when she was little, and have been a lifelong fan of Victoria Beckham, so a focus on Harper was a natural progression,” she wrote in an email. “Harper is very popular because of who her parents are and their personal style.”