Water deliveryman Zhang Feng is worried about the 8-year-old son he left in the care of his parents in his hometown in Shandong province when he moved to Beijing.

"They love him, but they give him whatever he likes to eat or drink, including unhealthy stuff such as soft drinks," he said.

"It is very normal for couples in my age group, those born after 1980, to ask their parents to look after their child, since they are too busy with their jobs."