THE CASE of Walter Polovchak, the 12-year-old boy who doesn't want to go back to the Ukraine with his parents, is a perfect illustration of what can happen when the judicial system gets misused in matters that are and should remain the province of the family, not of the state.

It is easy to sympathize with Walter -- too easy. Not many people among us -- at least not many who know anything about it -- would choose life under communist rule over life in the U.S.A. But it takes only a minute's thought to realize how outraged Americans would be if the tables were turned, if Walter were the son of American emigres disillusioned with life in the Soviet Union. If that imaginary Walter announced that he preferred to stay in Russia and was granted "protection" from his parents' will by the Soviet courts, Americans would surely see him as nothing less than a political hostage.

Walter's announced reasons for wanting to stay in this country are his new friends, his school, his bicycle, the abundance of food he can afford to buy and rock-and-roll music. Yet his lawyer, commenting on the case, could remark, "I would have sympathy if the father had a legitimate reason for leaving."

Presumably there are legitimate reasons on both sides. One cannot help worrying, for example, over what Walter's fate will be if he returns to the Ukraine. But the point is that the choice is a personal family matter, one for the Polovchak family, not the American state, to make.

The state of Illinois, where this case is being tried, has rules to define the circumstances under which courts may remove a child from his parents' care. They are similar to the laws of most other states. A child can be taken into the custody of the state if his parents don't want him, if they physically abuse him, if he is judged to be "beyond their control" or a danger to society. Ordinarily, a child who runs away to his cousin's apartment because of his parents' plans to move is not considered to be in any of these categories.

Walter's case is obviously more complicated, but it is a fact that Mr. and Mrs. Polovchak have done nothing to prove themselves unfit parents in the eyes of the law or to justify the need for court-ordered psychiatric evaluations. Nothing, that is, but to prefer life in the Soviet Union to life in Chicago. It is hard to avoid the conclusion that the court's interest in this case is a political one. And it should be obvious that there is an enormous potential for abuse when family courts begin making political judgments.