However, their journeys, which can last weeks or months, have sometimes been incompatible with their children's schooling. Wu recalls when Lindesay let their elder son ask for leave from his primary school so they could go to New York for a 45-day Great Wall lecture tour. The son missed his final exam, which upset Wu. "The family had a strong smell of 'gunpowder' at that time," she says.

Lindesay attaches great importance to learning out of the classroom, saying that children might score well on school tests, but that experience of the world outside, in distant lands, with different languages, scripts, political structures, and religious beliefs, is the real testing ground.

"You can only get street-wise out on the street. You can only get worldly wise when seeing the world," he says.

Years ago, such thinking was in stark contrast to that of most Chinese parents, who put their children's classroom study first and worried that too much travel would delay schoolwork and lower grades.

Children in this international family did not have the same pressure to perform on school tests, but they had "homework" on the road.

Wu asked her sons to write travel diaries, collect tickets, draw maps and summarize travel tips. She says such habits, though they might not directly improve test scores, will pay dividends in later life.

These experiences certainly shaped their sons' characters and influenced their chosen study at university. One read world history, the other international relations.

Another obvious payoff of their travels is that their two sons, now adults, also share an interest in historical monuments, and the Great Wall in particular. More than four years after graduating from university, the elder son Jimmy has a portfolio of video work about heritage protection, some of which has been broadcast on the BBC and in Chinese media. He and 21-year-old brother Tommy are now planning to follow in their father's footsteps with a new 4,500-km hike on the Great Wall.

"My parents view the world as a big classroom, and my brother and I are the biggest beneficiaries," Jimmy says.