Gary Coleman Meets a Child Molester? It doesn't sound like a very promising prospect, and viewers of last week's installment of the NBC family comedy "Diff'rent Strokes" may have been shocked to see the subject brought up, especially when it involved one of the nation's most beloved tots. But the conclusion of this two-part special episode, tonight at 8 on Channel 4, reveals it to be a calm, careful and intelligent treatment of a difficult and potentially traumatizing subject.

There seems little possibility that watching this program would do children harm, and considerable likelihood it could do them good. As with last week's show, actor Conrad Bain is seen out of character in a preface advising that families watch the program together, then discuss it afterward.

As skillfully written and directed by Blake Hunter and Gerren Keith, "The Bicycle Man" finds little Arnold (Coleman) and his pal Dudley (well-played by Shavar Ross) all but in the clutches of Mr. Horton (Gordon Jump), on the surface a jolly old sport, beneath the surface a disturbed and conceivably injurious man. The script is written so as to show him some compassion but portray him as a clear and present danger.

Tonight's episode begins exactly where last week's ended, with Arnold and Dudley being entertained by Mr. Horton in the back room at his bicycle shop. He's a little too friendly. He plies the boys with wine, Boston cream pie and dirty cartoons. Arnold has the good sense to run home, but Dudley hangs around, and Arnold's adopted millionaire father (Bain) calls the police.

In the denouement, a police detective doles out the hard information about child molesters. "In their own strange way they actually love children," he tells the assembled household. He also notes that some parents, wrongly in most cases, "actually blame the children for what happened." Arnold asks, "What is this world coming to?"--a question that's never out of line--and says, "Look, I'm only 11 years old. Should I be hearing all of this?"

Yes, Arnold is told, he and, by implication, the children who watch this extremely popular show, should. It can be argued that situation comedy is not the place for such topics, but that argument doesn't hold up very well after the precedent set in the '70s by Norman Lear, whose Tandem Productions makes this show. "Diff'rent Strokes" has previously dealt with such topics as teen-age drinking, sex and female independence. On March 19, Nancy Reagan will appear as herself on a "Diff'rent Strokes" episode about teen-age drug abuse.

And earlier this season, Gary Coleman proved yet again what a trooper he is with an outstanding performance in a touching episode in which Arnold visited the grave of his dead mother. Coleman is a natural wonder, but the time seems propitious to note that the cast of "Diff'rent Strokes" (Bain, Coleman, Todd Bridges and Dana Plato) has made itself one of the most appealing and human families in contemporary fictional television. A very good show appears to be getting even better.