SICHUAN, China (CNN) -- Li Yunxia wipes away tears as rescue crews dig through the ruins of a kindergarten class that has buried her only child -- a 5-year-old boy.

Other parents wail as soldiers in blue masks trudge through the mud, hauling bodies from the rubble on stretchers.

"Children were screaming, but I couldn't hear my son's voice," she says, sobbing.

This grim ritual repeated itself Thursday across southwestern China, as thousands of mothers and fathers await news about their sons and daughters. Watch parents' anguished vigil »

The death toll from Monday's massive earthquake could be as high as 50,000, according to state-run media. Map »

The grief is compounded in many cases by a Chinese policy that limits most couples to one child, a measure meant to control explosive population growth.

As a result of the one-child policy, the quake -- already responsible for at least 15,000 deaths -- is producing another tragic aftershock:

Not only must thousands of parents suddenly cope with the loss of a child, but many must cope with the loss of their only child.

China's population minister recently praised the one-child rule, which dates to 1979, saying it has prevented 400 million children from being born.

Some wealthy families ignore the order, have more children and pay a $1,000 fine. In rural areas -- like earthquake-devastated Sichuan province -- families can petition for an additional child, but there's no guarantee the authorities will approve the request -- they usually don't.

That reality has cast parents like Li into an agonizing limbo -- waiting to discover whether their only child is alive or dead.

Thousands of children were in class when the temblor hit Monday afternoon. Many of their schools collapsed on top of them.

In Dujiangyan City, more than 300 students were feared dead when Juyuan Middle School collapsed with 900 students inside. A similar number died at the city's Xiang'e Middle School. See images and video from the quake zone »

Now parents cluster outside collapsed school buildings, held back by soldiers in some cases as rescue crews search for signs of life.

"Which grade are you in?" a rescuer asks a trapped child in Beichuan County.

"Grade 2," comes the answer.

"Hang on for a while," he says. "We are figuring out ways to rescue you." Watch children rescued from the rubble »

The child is pulled from the rubble a short time later.

For every child saved, though, many more are lost.

Many are missing at a middle school in the city of Qingchuan. The scene is devastating at Juyuan Middle School, where sorrow seems endless. See photos of rescue efforts »

"There were screaming parents, and as the bodies would come out they were trying to identify whether it was their child or not," said Jamil Anderlini of London's Financial Times. "And once they -- the parents -- realized it was their child, obviously they collapsed in grief."

All About China • U.S. National Earthquake Information Center • Disaster Relief