IF YOU WERE SEEKING a mid-afternoon low yesterday, you might have tuned in on -NBC's "Reading, Writing and Reefers," a first-rate documentary about the chronic use of marijuana by children. The show offered all you ever wanted to know about marijuana, but were (wisely) afraid to ask: where it grows; how it works; how it is smuggled and sold; and, of course, the new language it has created, with the "tokes" and "pot-heads" and "copping a buzz." That language tumbles naturally,if drowsily, from the children on whom the show was centered. The testimony of four rosy-cheeked kids carried the show's clear message-that marijuana is dangerous. Had the kids said nothing, their lidded eyes would have borne the message for them.

Yet it wasn't so much what the children said or looked like that was depressing and alarming. It was the knowledge that as children they had found a fool-proof way to set life dreamily aside; in other wordso to effectively remove from their childhood all the encumbrances and difficulties that would challenge them into growing up. In a way, they resembled the children the Romantic writers used to pine for, the little fabrications, who strolled through a fabricated childhood where, as 15-year-old Keith said proudly yesterday, "nuthin" ever bothers you." Yet even Thomas De Quincey, the Romantic essayist who wrote the "Confessions of an English Opium-Eater," began his habit at 28.

Brian of Florida is 12. He began smoking pot regularly at 8 or 9. Now he is merely one of an estimated 4 million kids from 12 to 17 who use marijuana anywhere from often to daily. Their grades drop. They become "burned out," listless. To themselves they minimize the evidence of harmful effects. Yet the show presented argument upon argument against the use of marijuana by children-arguments quite apart from the question of decriminalization.

The only voice missing from these arguments was the voice of parents. When interviewed, they shrugged as if the children were plants themselves, sprouting out of control. And the show trod lightly on the issue of how responsible parents are to warn and guide their children. Yet who but the parents can bring the message of danger home with any clarity or force? "Reading, Writing and Reefers" was shown at 4 p.m. Whenever it is shown again-as it certainly should be-let it go on at 9 or 10, when the grown-ups are watching.