Becoming an entrepreneur wasn't part of her career plan. She studied architecture at Princeton, followed by a masters at Harvard and an MBA at Stanford. "Duck Duck Moose came out of my personal passion for education and technology," she says.

Hu Flexer, with her husband Michael Flexer and friend Nicci Gabriel, founded the company in 2008, bringing together shared interests in music, education, design and play.

"We were inspired by our own children," she says. "My older daughter was two-and-a-half at the time. She'd walk around holding up a book and singing song, which she loved. We thought it would be fun to create a touch-screen app as a new generation of the pop-up book. We saw how the touch screen made technology available to our own children in a way that was never possible for us."

The founders already had the skills for the job. Hu Flexer had worked as a design consultant at IDEO, the innovation consultancy founded by Stanford professor David Kelley, and as a product manager at software company Intuit.

Her husband Michael is a software architect with 20 years' experience at several Silicon Valley start-ups, and Gabriel is an award-winning designer.

"We had the skills to create everything in the app ourselves. Nicci did the illustrations, and all of us did the coding," she says.

For music, a key attraction of their apps, Hu Flexer played the violin, and Michael, who plays the cello, pitched in with his music trio. A friend and a cousin were recruited for the singing and speaking roles.

Needless to say, the Flexer children, Caitlyn, seven, and Sara, five, and later Gabriel's son Gray, two, were instrumental in testing the apps. "We even created our company website ourselves the night we submitted the app to Apple. It was a bootstrap company," Hu Flexer says.

All three held full-time jobs at the time, so work on Duck Duck Moose had to be fitted in at night and on weekends. Until last year, they were still working out of the Flexers' home.

Wheels on the Bus launched on the Apple app store in January 2009, and steadily garnered a huge following. It went on to win the KAPi Award for Best Children's App at the 2010 International Consumer Electronics Show.

It topped Apple's education section for nine months, earning a mention in Steve Job's keynote address at the iPad 2 launch.

The success of the app prompted them to quit their other jobs. Duck Duck Moose now has a stable of 17 apps and has received 15 Parents' Choice awards and 15 awards from . Relying largely on word-of-mouth publicity through mothers, schools teachers and social media, the apps have recorded more than 2.8 million paid downloads.

"It's been quite a journey for us," says Hu Flexer. "When we launched, it was early days in the app store, very different from today."

The mobile app creation industry is booming due to the proliferation of smartphones and tablets. Technology research firm Gartner has predicted that worldwide revenue from app stores will increase in 2013 by 62 per cent, bringing the total industry revenue to US$25 billion.

Duck Duck Moose owes much of its success to its child-centred design process. "We derive inspiration just watching how children play in the real world," Hu Flexer says. "There are standard buttons and interfaces that work for adults. But children can't read, and are not used to a mouse. Figuring out what works for them with the touchscreen was very new.

"Young children don't articulate a lot. So we study their facial expressions and their behaviour. They don't give you too much leeway if the design isn't good."

Frustration with an app can reduce a child to tears, as was the case with the unmovable stickers in their Draw and Tell app, which changed completely after it was tested.

She says their own children are like beta testers. "They can actually look for bugs for daddy to fix. We're teaching them to reproduce the steps, so that it's easier for the engineers to figure out," says Hu Flexer.

As their children grow older, the team is developing age-appropriate apps to keep up. Their recent Moose Math is for kindergarteners and first graders, filling a gap in a market between apps that are simply entertaining and what Flexer calls the "drill and kill" variety.

"We wanted to bring in not only colourful illustrations and the fun of learning, but also pedagogically sound content."

Hu Flexer acknowledges that apps should not replace drawing on paper or playing outdoors.

"What matters to me is the quality of the experience they're having with the touch screen. If they're singing songs, telling a story, learning phonics in an exploratory and interactive way, it's different from passive screen time like TV," she says.

With a US$7 million injection in funding from Sequoia Capital, Lightspeed Venture Partners and Stanford University last year, Duck Duck Moose has expanded its team to include a small group of engineers in Shanghai.

"The funding has enabled us to hire experts in the field and create richer experiences. We have film quality animators helping us design characters, and talented illustrators and engineers," she says.

For example, Moose Math allows unlimited users so teachers can use the app to track each child through a report card.

Building a company from scratch, she says, has been "all-consuming. In some ways, it's great to share that with your spouse. If we were both working at different start-ups I don't know how often we would see each other."

"We share a calendar both at work and at home and trade off days to leave early. We try to be home for dinner and work after the kids are asleep. It helps that we're doing something that our kids understand," she says.

One goal is to build Duck Duck Moose into a suitable environment for working parents. "A lot of people who share the same values as we do are parents. We want to make it a place where everybody works hard, they have interesting work, and they can go home to their kids and finish later if they have to," she says.