"The Kid From Nowhere" is another one of televison's "serious" dramas that make an attempt to examine the lives of those of us who have to carry special burdens.

With a fine cast headed by some of TV's most likable actors -- Susan St. James, Beau Bridges and Loretta Swit -- tonight's two-hour NBC movie (Channel 4 at 9) at times teeters on the brink and almost slips into soap-opera sentimentality. But some good performances and moments of honesty save it.

This is the story of Samantha Kandal (Susan St. James), a young mother with a retarded son, who nearly ruins her own life and that of her son, John, in her efforts to protect him from the rough-and-tumble of life, both physical and emotional. Finally, the coach at John's school (Beau Bridges) persuades her to let the boy train for the Special Olympics. This gives John his first experience of striving and accomplishment.

It well could have been a sentimental wallow as a romance develops between the coach and harassed mother. But St. James, with the charm of McMillan's wife when she is not being obsessively overprotective, and Bridges, with just the right measure of understanding, manage to keep sentiment in hand. Swit is fine as Samantha's swinging but supportive neighbor.

The role of John is played by l2-year-old Ricky Wittman, himself a victim of Down's syndrome. Some of the brightest moments on the screen come when Ricky's face lights up with a child's joy and wonderment as he watches a sparkler erupt on a cake or as he is awarded his trophy as the most inspirational athlete at the Special Olympics in Los Angeles.

In one episode that keeps the script honest, Samanatha invites Howard, her latest serious boyfriend (played by Rene Auberjonois), to dinner. It would have been easy to portray him as a villain without feeling. But Howard is portrayed as a warm human being who genuinely cares about Samantha and doesn't want her to ruin her life.

When he tries to be sympathetic and she retorts bitterly about "cheap psychology," he replies with simple dignity: "I didn't deserve that." And he didn't. You do have to wonder why Samantha didn't warn him that her son was retarded and might throw food in his face at the table.

I suppose some of these inspirational dramas may help our understanding of those with special problems in life. But I'm not sure that the mother of a retarded child will always find a Beau Bridges as the coach at her son's school who understands and helps her with her life.