"Broken Promise," the CBS movie at 9 tonight on Channel 9, lives all too literally up to its title. After a powerful and succinct opening scene sets up intriguing and harrowing possibilities, the script proceeds to ignore most of them in favor of heroine-on-the-ice-floe mellerdramatics.

The first five or 10 minutes are grabbers, though. A family of seven is driving a run-down Chevy across America in 1964. The father, we learn, has spent eight years in prison, and not all the kids are his. The mother has that lived-on look. The children are complaining in the back seat.

He stops at a gas station. The kids are sent off to the nearby E-Z Stop Cafe for cheeseburgers. The father suggests he and the missus leave the children behind and continue on to California alone. And they do.

The adventures of the abandoned children that follow seem only occasionally plausible and once or twice preposterously trumped-up. But two key performances sustain the film through trying times: Chris Sarandon as a bighearted child welfare worker who tries to find all five kids a common home, and Melissa Michaelsen as the oldest child, a stalwart, take-charge 12-year-old named Patty.

In this child's eyes one sees not the typical tot starlet but an instinctive, assured and beautiful young actress; she takes over the family and she takes over the film, even though Stephen Kandel's patently inadequate script is content merely to lurch from floe to floe, shamelessly but not very effectively. Michaelsen could be the unspoiled answer to a Tatum O'Neal or a Brooke Sheilds, though how long she can hold on to her ingenuous credibility in Hollywood is open to question.

Naturally the villain of the piece is The System, in this case a pompous swine of a bureaucrat -- head of the local welfare board -- played by George Coe; he's even more of a rotter than the Snidely Whiplash who tried to prevent Richard Thomas from adopting a little boy in "To Find My Son," another Man Vs. System potboiler, on CBS last October.

The children are split up, and Patty gets tossed first into a foster home from which she runs away, then into a loony bin for tests. Naturally, while there, a deranged old crone breaks into her room in the middle of the night. Then the girl is thrown into a juvenile detention center; naturally, while there, she is involved in a telegenic wrestling match with another young girl; director Don Taylor seems to have a particular affection for this scene.

A sort-of-happy ending comes about on Christmas Eve, no less, and a voiceover explains that since the period depicted in the film, changed have been made in adoption procedures, so that this small-fry version of "Snake Pit" turns out to be TV's favorite kind of searing expose: a searing expose of conditions that no longer exist.