A mentally ill or retarded child has a right to have a "neutral fact finder" decide if his parents may commit him to a public or private institution, the Supreme Court ruled yesterday.

The fact finder may be an admitting psychiatrist who works for a state, the court held in a pair of 6-to-3 decisions in cases from Georgia and Pennsylvania.

The decisions rejected contentions that the "massive curtailment of liberty" resulting from commitment gives a child a constitutional right to a hearing at which his representative would present evidence.

In doing so, the court preserved the existing commitment procedures of more than 30 states, including Maryland and Virginia, and of the District of Columbia.

Chief Justice Warren E. Burger, in the opinions for the court, said that, in addition to the fact finder, children must have the protection of periodic reviews of the need for commitment.

The admission of a child to an institution at the insistence of his parents long has been a troubling issue in the courts.

The basic presumption always has been that mothers and fathers speak for their children. But the strong pull of tradition has been met by the tug of reality: some parents are unfit or sacrifice a child's interests to their own.

The Supreme Court avoided the issue in 1977, when it sent the Pennsylvania case back to the state's highest tribunal. It avoided the issue again last year, when it set the Georgia case for a second round of arguments rather than decide it.

The issue had divided professionals who care for children. Yesterday's outcome was sought by the American Psychiatric Association - and opposed by the Children's Defense Fund (CDF), a Washington-based nonprofit group, and several organizations of professionals providing mental health care.

CDF counsel Daniel Yohalem told a reporter that the rulings, by upholding "the predominant mode" of admission, will work "to the detriment of children."

He questioned the ability of state psychiatrists to be "neutral" fact finders. At the same time, however, he said he was encouraged by the court's statement that the states are free to require hearings, if they care to.

Under a 1978 local court ruling in a case initiated by the CDF, juveniles 15 or younger in the District of Columbia are entitled to hearings before commitment to a public mental institution. Maryland and Virginia have similar requirements.

Yesterday's decisions reversed a panel of three federal judges, including now-Attorney General Griffin B. Bell, in the Georgia case, and the state supreme court in the Pennsylvania case.

In both cases, the chief justice found that the states' procedures struck acceptable balances between rights and needs in tension with each other.

The law's concept of the family "historically . . . has recognized that natural bonds of affection lead parents to act in the best interests of their children," Burger said. That parents at times act contrary to those interests "creates a basis of caution, but is hardly a reason to discard wholesale . . . pages of human experience . . ."

Yet, he said. "We have recognized that a state is not without constitutional control over parental discretion in dealing with children when their physical or mental health is jeopardized."

Burger rejected the argument that a formal pre-admission hearing is warranted in the context of a child's interest in liberty on the one hand and the likelihood of parental abuse on the other.

The argument "sweeps too broadly," he said. "Simply because the decision of a parent is not agreeable to a child or because it involves risks does not automatically transfer the power to make that decision to some agency or officer of the state."

At the same time, Burger wrote, "Parents cannot always have absolute and unreviewable discretion to decide whether to have a child institutionalized."

As for the state, he said, it "obviously has a significant interest in confining the use of its costly mental health facilities to cases of genuine need," in not putting needless obstacles in the path of those seeking aid, and in using scarce professionals for diagnosis and treatment rather than pre-admission "time-consuming procedural minutes."

Putting all of these factors into the scales, Burger concluded that the "neutral fact finder" should determine whether the medical requirements for admission have been met.

A staff physician can be that fact finder "so long as he or she is free to evaluate independently," he said.

Burger spoke of the need to avoid an "adversary contest" between parents and child. CDF lawyer Yohalem said most hearings aren't adversarial.

A dissenting opinion in eachcase was written by Justice William J. Brennan Jr. and signed by Justices Thurgood Marshall and John Paul Stevens.

Brennan agreed with the majority that the question of the frequency of post-confinement hearings properly could be deferred. But "the right to at least one" such hearing "can and should be affirmed now," he said.

He termed unconstitutional Georgia's failure to grant post-confinment hearings to wards of the state and Pennsylvania's provision of "neither representation nor reasonably prompt post-admission hearings to mentally retarded children 13 years of age and younger."