When a young child sees a television advertisement for a sugary cereal and is then told by a parent that the cereal is not good for him, the child will begin to mistrust the parent, a prominent child psychologist said today.

Testifying here at the second of Federal Trade Commission hearings on proposed limits on TV advertising aimed at children, Prof.Robert Libert of the State University of New York at Stony Brook said children under eight are not likely to make the advertiser the target of skepticism when another authority figure, like a parent or teacher, challenges a television ad.

In contrast, a spokesman for Mattel Inc., a major toymaker and TV advertiser, said that children's advertising is not misleading or decepitve, and to ban such advertising would, in fact, violate the rights of children. He said, too, that such a ban could increase toy prices by up to $840 million annually by eliminating economies of scale in their manufacture and distribution.

Liebert testified that "young children may become skeptical of their parents, pre-school teachers, or any others who disparage sugared food products or a poorly made toy promoted artfully.

"Mistrust results," he added, "when legitimate authority figutres -- such as parents -- are implicitly silenced or discredited, as they are if they pit their meager persuasion techniques against the might of television advertising directed at their young children."

Liebert was testifying in the support of FTC staff proposals to ban TV advertising aimed at children under eight, and place limits on the advertising of sugary foods directed at children under 12.

Citing what he calls "unambiguous" research, Liebert said "children up to about age seven are simply unable to understand selling intent regardless of the communication techniques used in advertising directed at them. It is therefore inherently unfair to direct television advertising at this age group."

Liebert also said that "it is horrible to think of children as targets, as the advertising industry does. Children cannot grasp the fact that they are exposed to material that is calculated to interest them in products."

For their part, industry spokesmen have protested that their ads are honest, and that a ban on any children's ads would violate their First Amendment rights.

Mattel spokesman Michael Weinstock testified that "in addition to affording children special protection, the law is obligated to protect their inherent rights. We ask the commission to consider the proposition that because toys are designed and manufactured primarilyfor children (many of whom cannot read), to be used primarily by children, and to be enjoyed primarily by children, there is no product in the American economy for which the dissemination of information to children by television is so compellingly justified."

"Our position simply stated," Weinstock added, "is that the proposed ban is unconstitutional, economically injurious and unnecessary. Therefore, it will do more harm than good"

Spokesmen for Mattel and others in the industry have estimated that toy costs, should a ban on TV advertising be imposed, would rise about 15 to 20 percent, or about $840 million annually.