AWEEK before, Soviet armor and paratroops had pulled out of the area in Afghanistan's Ningarhar Province. Behind them they left a variety of toys, scattered by a riverbank. Among them: a bright red plastic truck. A 14-year-old Afghan boy made the mistake of grabbing it. It exploded in his hands. It was one of the booby-trapped toys the Soviets are using as weapons in their war against the civilian population of Afghanistan that opposes Soviet occupation.

In this particular instance, the boy was fortunate. The booby-trapped toy was defective. He retained his fingers. Other children have been less fortunate. They have lost hands, or even been killed.

This is part of the toll the Afghan war - a war that has taken a million civilian lives and caused more than 3 million refugees to flee Afghanistan - is inflicting on children.

The story of this toll, and the suggestion that children are sometimes singled out by the Soviets for intimidation and terror, has been collected in a report by the Center on War and the Child, an Arkansas-based foundation. According to Richard J. Parker, the organization's director, the center has two other reports under way, one on the role of children in the Iran-Iraq war, and another on the impact of conflict in Uganda on children. ``Children,'' he says, ``shouldn't be the victims in wars fought by adults.''

Mr. Parker says the center accepts no government money for its nonprofit operations and espouses no particular ideology. Thus while the Soviets bear the brunt of its criticism for cruelty to children in Afghanistan, the use of children by the mujahideen, or freedom fighters, is also chronicled disapprovingly.

Citing a study prepared for the United Nations Commission on Human Rights, along with a variety of eyewitness accounts, the report confirms the use of booby-trapped toys and soap against children, the conscription and deportation of children to the Soviet Union, and the indiscriminate bombing of villages resulting in the death of children.

There are also accounts of the execution and burning of children which are too graphic to be recounted.

Perhaps one of the most cruel and morally offensive Soviet tactics against Afghan children, says the report, is the use of antipersonnel mines constructed to resemble toys. The toy bombs have been disguised as dolls, chewing gum, pens, trucks, combs, and other common objects. When picked up, they explode.

The Center on War and the Child says that in a struggle for the loyalty of Afghan children, thousands between the ages of 6 and 9 have been transported to the USSR for up to 10 years of study. The purpose of this is to create a cadre capable of leading a future communist Afghanistan.

The report makes clear that on the other side of the war, among the mujahideen, there is a corresponding attempt to indoctrinate young children.

In the schools across the border in Pakistan, crowded with Afghan refugee children, regular lessons are heavily laden with political education. Says one schoolteacher: ``We never just teach them that two plus two makes four. We say that two dead Russians plus two dead Russians make four dead Russians killed by the mujahideen.''

With the exception of the Iran-Iraq war, where thousands of young boys have been sent on suicidal human wave missions, the calculated victimization of children in Afghanistan is as gruesome a use of children in warfare as one can imagine.

So far the Arkansas institution chronicling this cruelty is not impressed by the character of Western protest. ``Tragically,'' it concludes, ``the Soviet Union has been permitted to engage in a policy of genocide directed at children of the resistance with little challenge or moral condemnation from the United States and the rest of the world.''