A powerful antiwar poster that Americans saw everywhere in 1970 showed bodies heaped in the Vietnamese village of My Lai. The caption was terse, taken from an inquiry into that massacre perpetrated by Americans: ''Q. And babies? A. And babies.''

No such posters draw attention to the ghastly, deliberate crippling of children by Soviet invaders in Afghanistan. Indeed, having grown skeptical of Presidential anecdotes, some Americans may wonder if Ronald Reagan was talking through his evil-empire hat when he accused Russians of sowing insurgent areas with bombs disguised as toys. The evidence isn't anecdotal. The evil is real.

It lies exposed in a report to the United Nations Commission on Human Rights. This inquiry, the first ever by the U.N. into abuses charged against a Communist state, seems to have been scrupulously conducted by an Austrian legal expert, Felix Ermacora. Barred from Afghanistan, he gathered incontrovertible testimony of the slaughter of civilians from Afghans who fled to Pakistan.

The report asserts: ''The most horrible type of incident was that caused by the explosion of anti-personnel mines and especially of children's 'toys.' Many witnesses testified that children had been very seriously wounded, having their hands or feet blown off, either by handling booby-trap toys they had picked up along the roadway, or by stepping on them. . . .