“I get emails that bring tears to my eyes,” Choi says. “I received an email from a mother of twin girls who said: ‘One of my girls reads your e-book to her sister every night, and they love the songs.’ I didn’t understand at first, but then I found out that one of the girls is blind.”

Choi attended high school in Hong Kong and Britain, and in 2005 moved to California to study music and business administration. While in the US, she met the Academy Award-winning composer Hans Zimmer, whose film scores include those for Pirates of the Caribbean, Inception and The Dark Knight trilogy.

Choi had identified that mobile technology was revolutionising the way children were learning, and increasingly, teachers and parents were turning to mobile apps to provide new, engaging ways to introduce subjects to youngsters.

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And the sector is growing rapidly, says Maria Soledad Riestra, the Argentinian founder and director of Ecoed, a Hong Kong-based environmental education company.

“More people and organisations are tapping into this space as a way of reaching out to others, especially the younger generation,” Riestra explains. “Communicating about environmental issues is super difficult. It’s doom and gloom mostly, and people don’t want to hear about it. That’s why it’s important to find other ways of engaging people.”

On her return to Hong Kong, Choi founded her company, Happy Diamond Music, and set to work. A committed environmentalist, Choi thought she could find a new way to tell children about the dangers of climate change.

“Everyone has their own way to contribute to the fight, and I want to use my music to do that,” she says.

Her first e-book app, Kakamega, released in 2014, tells the story of three children who venture into the rainforest on a quest to find a flower with magical healing properties.

Choi composed 90 minutes of music for it, and each discovery the characters make is accompanied by a song.

“I wanted to make kids aware that if we keep chopping down the trees, we won’t have a rainforest to turn carbon dioxide into oxygen – but I try to make it fun: that’s why all my apps have music all the way through, so kids can sing along,” she says.

To generate empathy among her young readers, she tells part of her stories from the point of view of animals, with talking polar bears and singing parrots making appearances.

Choi, who does not have children of her own, gets many of her ideas from her own audience.

“I interview kids and get a lot of inspiration from them,” she says. “They have no self-imposed restrictions. If you ask them a question, they respond with the most creative, unexpected ideas.”

The figures suggest that her approach works: on its release, Kakamega reached No 11 on Apple’s iOS app chart in the book category.

“Ninety-five per cent of my downloads come from schools overseas – in the United States, Britain and Australia,” Choi says. “That’s really encouraging.”

She has no idea how they find her apps, but clearly, teachers are out there looking. “I know when a school has found one of my apps, because they bulk-download it 500, 1,000, 2,000 times in one go!” she says.

A year later, the sequel, Migalolo, performed even better, hitting No 4 on the iOS chart with well over 100,000 downloads – at one point beating such apps as Amazon’s Audible and even the King James Bible. Migalolo focuses on the oceans, with the script and songs themed around marine conservation.

Choi’s third e-book, Miniwalla, about a wildlife sanctuary in Indonesia, came out in 2017.

Helen Li, a mother of three who found Choi’s work via the App Store, says the apps are the ideal fit for life in 21st century Hong Kong.

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“The world is changing so fast, and we live with these devices around us – so why not make use of them in a positive way?” Li, 40, says, adding that her children, aged 11, nine and six, are particularly keen on the songs, but that the serious message is not lost on them.

“The apps are fun, but the stories are meaningful, so it’s a great way to teach kids about why we should take care of our planet,” she says.

On how Choi can go on without turning a profit, she says her income comes from property investments in Hong Kong and overseas. This has freed her to pour her energies into things she cares about – such as environmental awareness.

Choi is completing work on her fourth e-book app, which has a stronger nod to Chinese culture than her previous work, and is slated for release in early 2019.

Titled Gulangyu after the historic island just off Xiamen, “it’s the story of a Chinese girl who travels the world with her trusty, multilingual parrot and baby dragon, to discover the causes of climate change and save the Earth”.

“Perhaps one day, one of my stories will get made into a movie, or a TV show. But I’ve received great feedback from teachers and parents, and that’s enough to make me a happy girl.”