China's economic miracle has lifted about half a billion people out of a 'dollar-a-day poverty' in little more than three decades. It is an unprecedented success story. Australia is obsessed with monitoring the economic health of the rising superpower as it directly impacts on our growth and prosperity. But people rarely look at the social costs of rapid industrialisation and urbanisation of the world's most populous nation. The left behind kids is one biggest and most urgent issues facing China today, as China correspondent Matthew Carney reports.