Holiday heartwarmer season gets off to an extremely tolerable, even felicitous start this week with two exceptional movies on CBS, both based on true stories and both concerning selfless adults who took up the unassailable cause of kids: "The Children Nobody Wanted" on Saturday and, tonight at 9 on Channel 9, "The Marva Collins Story."

A radiant and two-fisted performance by Cicely Tyson in the lead role gives "Collins Story" added weight. Tyson plays the Chicago elementary school teacher whose disenchantment with the school system and the defeatism of those around her ("These children don't want to learn," scowls one teacher) inspired her to start her own school in her own home.

The film fits into the category of Fighting the System sagas, now the dominant strain in serious TV movies ("She rebelled, and challenged the system," says Ed Asner in the opening narration), but it's elevated by Tyson's performance, by the absolutely disarming and beautifully cast kids, and by writer Clifford Campion and director Peter Levin, who pull off the tricky feat of effectively dramatizing the joy of learning. Together they make "Collins Story" the best public relations for teachers since Martin Ritt's "Conrack."

As the film opens, the ghetto school where Collins teaches is all shrieking bedlam. At home, Collins' husband (Morgan Freeman, solid as always) gives one of the couple's own children some money for school, but only with the admonition that should anybody try to take it away from him, "you give it up, understand?"

Soon Collins leaves the school and takes on the bureaucracy, but these are the film's worst scenes. Going mano a mano with a clerk in a government office seems trumped-up conflict, and Campion doesn't bring it off. The film is at its best in the classroom, especially during one bravely extended sequence depicting the first day in the new school. The camera finds Tyson alone in the room, silent, waiting; with no dialogue, she conveys every nuance of apprehension that Collins must have felt at the moment, but her own raw pluck wins out, and the scene ends with her clasping her hands in confident elation.

She gets learning-disabled children to read and comprehend far beyond their usual performance by making them appreciate their own worth and potential: Tina, heartbroken when another teacher had told her she'd "failed"; Martin, who is tricked into learning with a book from Teach about his favorite subject, airplanes ("To Martin. Fly high. Mrs. Collins"); and Eddie, an unleashed whiz kid whose mother removes him from the school because "my child ain't no laboratory rat."

Mrs. Collins tells them about Socrates and Einstein and Victor Hugo and that "You children were born to win." When asked about their ambitions, one young lady announces, "I want to be the first black woman on Mars." And a skillful classroom montage, accompanied by one girl's reading of Kipling's poem "If," ends with a shot of a little boy who's fallen asleep with "A Tale of Two Cities" on the bed beside him.

Storm follows strife, but the school pulls through. In the closing narration, we are reminded that President Reagan offered Mrs. Collins the job of Secretary of Education but she declined so that she could stay in her community. But the emotional climax of the film comes earlier, when the kids throw their teacher a classroom surprise party and one of them reads her an original and appreciative poem.

At this point, it's every viewer for himself when it comes to controlling the old waterworks. "The Marva Collins Story" is about as wonderful as this sort of thing can get.