Lawyers for children in mental institutions urged the Supreme Court yesterday to void laws that permit parents or guardians to commit children indefinitely and without the hearing procedures granted adults.

The court was asked to uphold lower court rulings in Pennsylvania and Georgia, where state mental health procedures were found to be unconstitutional violations of juvenile patients' rights.

The decision could have national significance, because most states now have similar statutes regarding placement of juveniles, according to attorney Daniel Yohalem of the Children's Defense fund, which has filed a friend-of-the-court brief on the children's behalf.

In a similar case recently, the U.S. District Court for the District of Columbia struck down a city law allowing voluntary parental commitment of children under 18 to St. Elizabeths Hospital. The Sept. 25 order mandates: legal counsel for any voluntary youth admission, placement in facilities less restrictive than the hospital where possible, and independent psychiatric examination before administration.

That order was agreed to by lawyers for the Department of Health, Education and Welfare, which administers St. Elizabeths, and by the District and the Children's Defense Fund, which brought the lawsuit in 1974.

Pennsylvania and Georgia argued before the Supreme Court yesterday, however, that such rulings will prohibit needed mental health care, intrude on parental prerogatives and cost huge amounts.

The Pennsylvania case touches both mentally ill and mentally retarded children in public and private facilities. Although the state has repealed its statute allowing indefinite commitment of youths by parents since the suit was filed, the U.S. District Court for eastern Pennsylvania in May declared the amended laws unconstitutional.

The lower court ordered all confined juvenile released from state facilities and re-admitted with provision for lawyers, two hearings on their confinement, and the right to seek release from the facility.

The state contends this would be an enormously costly procedure.

The lower court wrongly assumed that earlier state commitments of youths were erroneous, and that parents and children have "an inherent conflict of interest," Pennsylvania argued. Deputy Attorney General Norman J. Watkins said the ruling "ignores the traditional role of the family" and of parents in determining their offspring's best interests.

But Philadelphia attorney David Ferleger, representing the 12 children named in the suit, countered that there is no historical right of parents to confine their children without due process. The state law allowing parental confinment was enacted in 1966, Ferleger said.

A friend-of-the-court brief by the federal government on behalf of the Georgia plaintiffs argues that there is a "constitutional defect" in current state procedures, but it urges the Supreme Court to send the case back for further consideration by the lower court of need for facilities other than hospitals and periodic reviews of juvenile patients' need for confinements.