A reporter shows Wang Huanhuan, 8, from Guizhou province, the video she made for the children during their Beijing trip.

"The parents are unable to come home. But, flights from small to big cities usually have seats. Why not make full use of these resources and take the children to their parents?" says Wang Yong, founder of Shunfengche, co-initiator of the campaign that brought Wang Jing to Beijing.

Shunfengche, which translates as Free Ride, is a Beijing-based NGO that helps people get free rides by matching those in need via text messages, mobile messaging app WeChat and its official website.

The campaign helped more than 10,000 people return home for free in 2012 and 2013. This year, it has helped about 20,000 people to travel home for free. Among them, about 1,000 were migrant worker parents who returned home to be with their children.

There are 61 million "left-behind children" like Wang Jing in China, according to estimates from the All China Women's Federation. That means one in every five children in China lives without one or both parents, who have to migrate to urban areas far from home to earn a living.

In less developed areas in western China like Guizhou, the percentage is even higher. It is estimated that more than 40 percent of the children in Guizhou are left-behind by either one or both of their parents. These children, cut off from their parents sometimes for years, may have difficulty connecting with others emotionally.