CBS recently reran its movie about a child molester, "Fallen Angel," because, the network said, of its social importance and impact. Also--the network didn't say--when the movie first aired in February 1981, it logged the biggest rating of any TV movie that season. "Fallen Angel" did deal, and responsibly, with a social problem, but when you deal with a problem, that means you get to depict it. How many people tuned in to "Fallen Angel" out of concern, and how many tuned in for the cheap thrill of implicit titillation? We'll never know.

The producers were smart, though; they legitimized their movie in advance by hooking into the Washington-Hollywood connection. A producer gets on the W-H circuit by garnering some official support from Capitol Hill before a controversial problem-drama is shown. Producer Lew Hunter did it with "Fallen Angel," and Moonlight Productions has done it with "In the Custody of Strangers," a glum shocker about the incarceration of a 16-year-old boy in a county jail. It airs on ABC (Channel 7) tonight at 9.

"Strangers" is competently done alarmism, full of melodramatic turns of event. When a teen-age boy is picked up for drunk driving and tossed into the pokey, his father, phoned at home, tells the police a night in jail might do the troublesome kid some good. But no sooner is the kid locked up than he is the target of a homosexual pass thrown by a potbelly in an adjoining cell.

Because the boy clanks the attacker's head against the bars until the fellow has a concussion, he is charged with assault. Eventually, thanks to bureaucracy and blunderers in the judicial system, the boy's one night in jail turns to a cruel and unusual 40. On one of those nights, he slashes his wrists. A prologue states that the story is fiction but that "479,000 juveniles are detained in jails" each year.

And there you have your social significance, erstwhile or not. Moonlight contacted the office of Sen. Arlen Specter (R-Pa.), chairman of the subcommittee on juvenile justice, to get the film seen in Washington and precipitate word of mouth. If there had been hearings under way, that would have been ducky for the producers, but since there weren't, they settled for a Specter-sponsored cocktail party.

In this town, more probably happens at cocktail parties than at hearings anyway.

The film itself is gripping; any teen-ager seeing it will be properly terrified. Actor Martin Sheen is never believable as a blue-collar worker (he looks like an unusually shifty lawyer), but you do believe that he could be the father of Emilio Estevez, who plays the son, because, in Real Life, he is. As the days go by and the boy's mental condition deteriorates, his mannerisms and appearance grow more and more similar to his father's. Perhaps the moral is that if you spend enough time in jail, you will turn into as gloomy a gus as Martin Sheen.

Jane Alexander does her standard ooze of concern as the boy's mother--another droningly monotonous Alexander performance. The real actor on the premises is the unfailingly reliable Kenneth McMillan, who plays the sheriff not as just crusty-but-lovable but as a seasoned, determined social realist. Jennifer Miller's screenplay, directed with more than the usual TV-movie flair by Robert Greenwald, makes its token attack upon "the system," but the way the story is structured, everything would have been fine if the father weren't such an irresponsible and callous hothead.

People watch these movies in droves, but do they really do any "good"? We asked a CBS publicist if there had been any substantive response to "Fallen Angel" the first time it aired. She said, "I do believe that a congressman put something into the Congressional Record about it." Oh, wow. But producer Hunter said from his Los Angeles home, "There have been 12 recorded cases I know of where someone called someone in authority and said, 'There's a guy doing to me what the guy was doing to the little girl on the television show.' " He said, "Girls are turning in their pedophiles," and that this is a tribute to the show's effectiveness.

He also said that while "Fallen Angel" drew a huge audience, another prob-drama he did this year, "Desperate Lives," about teen-age drug addiction, flopped. Hunter wonders if topics that hit too close to home are still unwelcome in American living rooms. Not even access to the DC-LA pipeline is a guarantee of success--but, like they say, it couldn't hurt.