Many of ABC's "Afterschool Specials," produced for children, have had the ironic effect of making the network's prime-time fare look childish by comparison. Perhaps former ABC Entertainment boss Fred Silverman was aware of this when he personally stepped in to take "Roll of Thunder, Hear My Cry" out of development for the "Afterschool" spot and turn it into a three-part prime-time miniseries.

The story of a stalwart and resilient black family's fight against poverty and prejudice during the Depression starts tonight at 8 on Channel 7. The remaining two hour-long chapters will be seen Saturday at 8 p.m. and Sunday at 7 p.m.

It is easy to see the elements in this story - adapted by Arthur Heinemann from two novels by Mildred D. Taylor - that impressed Silverman, since promotional material from the producers, Tomorrow Entertainment, Inc., compares it to both "Sounder" and "Roots." The film never seems derivative, however, because it deals with a time, 1933, and place, Mississippi, still novel for television.

And of course as a positive portrayal of a brave black family, it could hardly be more unassailble, and it should have considerable enrichment value for black and white children alike. As a dramatic production, however, it would have benefited greatly from more care in preparation, more money spent on location details, and a director who could demonstrate more appreciation for the story's emotional power than does the indifferent Jack ("Airport 1977") Smight.

Films for television may not be a director's medium nor a place for expressionist experimentation, but Smight's minimal, diffident approach is unfair to the actors and the material. Fortunately, the actors keep the story movingly alive anyway, even when Smight is too lazy to get their faces in close-up or animate static scenes.

The characters who will prove most unforgettable, especially to young viewers, are the Logan family's four children - Cassie (who narrates the story), Stacey, Christopher John and Little Man. Now and then Smight manages to catch all four in a truly affecting, even heartbreaking image - as when they march down a muddy road toward school only to be splashed by the passing school bus reserved for whites, or when they one-by-one reject the second-hand schoolbooks, with "Race: Nigra" stamped on them, that are given them by the all-white board of education.

Episodically but effectively, Heinemann's script tells of obstacles surmounted and dignity maintained even in the face of repeated assaults from the small town's dominant white racist, who happens to have been given the last name of Wallace.

Claudia McNeil as the grandmother and Janet MacLachlan as the mother are both hearteningly and believably valiant, and so are Robert Christian as the father and Morgan Freeman as ebullient Uncle Hammer. But it's the kids who steal the show, and on whom the script should have concentrated even more.

When they line readings are stilted or limp, you forgive them, but no apologies whatever are needed for Lark Ruffin's portrayal of Cassie, who is 11 years old when the story takes place but has virtually become an adult by the time the year of ordeals is over.

Ruffin watches from the window as cars come in the night , and the look of bewilderment on her face id devastating. And it should be noted that not all the white southerners depicted in the film are the snarling sort; the most decent is expertly played by John Cullum, whose face is becoming so ruggedly Lincolnesque that you are tempted to salute it.

With more time, care and a director of some discernible passionate sensitivity, "Roll of Thunder, Hear My Cry" might have been as intimately impressive as the movie "To Kill a Mockingbird," but even with its shortcomings, it represents three hours one need not regret having spent with television, especially if they are spent by the family all together.

Two of the best TV specials of the '70s - one musical, one dramatic - will be rerun within the next few days. "America Salutes Richard Rodgers," the CBS spectacle on Channel 9 Saturday at 9 p.m., scored poorly in the Nielsens when first run on Dec. 9, 1976, even though it is a rich, stylish, fitfully rousing and gorgeously produced two hours.

"Our Town," the fourth and most nearly complete television adaptation of Thornton Wilder's landmark in Americana, will be repeated Monday at 9 p.m. on Channel 4. NBC's production of the Pulitzer Prize-winning play was first shown May 30 of last year and should become a TV perennial. This Year's presentation marks the 40th anniversary of Wilder's play. It will live to be at least 140.

The Rodgers show, produced and directed by the inexhaustibly inventive team of Gary Smith and Dwight Hemion, certainly proved a grand night of singing. Though the first hour may seem a bit ephemeral, and though Gene Kelly and Henry Winkler aren't especially inspiring in their portrayals of Oscar Hammerstein II and Lorenz Hart - Rodgers' two principal lyricists - the two hours are musically and photographically impeccable. The program is a stunner from beginning to end.

Highlights include a videotaped medley by "the people of America" (actually and obviously the people of California) who were stopped on the street and asked to sing Richard Rodgers song in order to prove the breadth of their appeal. This is a dazzlingly edited and scored segment which includes a shot of a policeman and his motorcycle (he sings "Oh, What a Beautiful Mornin'") that is a work of a neo-realist art in itself.

But then comes the real lollapalooza: A montage a trois featuring Peggy Lee, Lena Horne and Vic Damone trilling and interweaving 24 different Rodgers songs with such class, such pizzazz, such snap - you'd never guess this was, in effect, a lament for the American musical theater, which has fallen on considerably less melodic times since Rodgers became inactive.

During the period when "Rodgers" was originally shown, it was outrated by "The Captain and Tenille."

"Our Town" was produced and directed by another man of taste, George Schaefer, one of the few veterans from the golden age who'll still bother to give TV a tumble. He used minimal sets, no props and sound effects to be faithful to Wilder, and the production that results is a true beauty - nostalgia on a celestial level and an invigorating celebration of basic values and domestic truths.

Schaefer's cast includes Hal Holbrook as the stage manager, Ned Beatty, Barbara Bel Geddes, Ronny Cox, Sada Thompson and, as the two young lovers, Robby Benson and Glynnis O'Connor. Benson said later that, gosh, he really was in love with O'Connor, and it shows. Their graveyard scene together near they play's conclusion elevates tearjerking to a new art.

Bell Telephone, sponsor of the program, and NBC, a certain television network, refused to allow the drama to run one second longer than its allotted two hours, so John Houseman's portrayal of a local teacher had to be eliminated entirely from the tape, leaving an awkward edit. This is the only sour note o be sounded regarding this magnificent production. However, if NBC fades out the closing hymn in order to run a network promo on Monday night, it will prove itself as cold and heartless as corporations are often said to be.