The Labor Department yesterday proposed major changes in federal child labor regulations that affect 14- and 15-year-olds, including provisions that would allow the teen-agers to work longer hours and to accept jobs that previously were considered too hazardous.

The department also proposed easing the procedures that employers must follow when they hire full-time students at less than minimum wage.

If approved, both proposals would help the fast food restaurant and amusement park industries, which lobbied hard for the changes. Robert Neville, a spokesman for the National Restaurant Association, called the proposed changes long overdue and said they will make it easier for youngsters to find jobs.

AFL-CIO officials, however, called the proposals outrageous. "It appears that the Reagan administration has turned the clock back about 70 years," said Rex Hardesty, spokesman for the 14.9 million-member labor federation.

"This is just unbelievable," added Robert F. Harbrant, president of the AFL-CIO's Food and Beverage Trades Department. "At a time when there are 11 million unemployed in this country and unemployment among blacks is 18 1/2 percent . . . they the Labor Department now want to extend the problem even more by expanding a kiddie work force."

The proposals, the first major revisions in the child labor rules since 1938, would allow 14- and 15-year-olds to work as late as 9 p.m. on nights before school days and until 10 p.m. on other nights. Now, those youths must quit work at 7 p.m. during the school year and at 9 p.m. during the summer.

The new rules also would let the teen-agers work 24 hours during a school week instead of the current 18-hour limit, and four hours daily instead of three.

In weeks that school is in session for only part of the week because of holidays or vacations, students could work up to 36 hours. The proposals leave intact a rule that says teen-agers cannot start work before 7 a.m.

The proposals would drop job restrictions on several industries, particularly in cooking and baking jobs. Under the proposals, 14- and 15-year-olds could be hired to perform any kitchen tasks that did not involve handling hot grease, working over an open flame or using cooking containers without safety valves.

Teen-agers also would be allowed to operate switchboards and teletypewriters, wash and polish exteriors of trucks and buses, operate automatic data processing equipment, fold and sort clean laundry in rooms where no laundry machinery is operated, work in self-service laundries and fill orders in warehouses.

Labor Secretary Raymond J. Donovan said the proposals do not change prohibitions on youth employment in particularly hazardous jobs. "These changes are designed to increase job opportunities . . . and to make it easier for employers to understand and comply with our rules," he said.

But Harbrant claimed that once the teen-agers were allowed in dangerous workplaces, they could be forced to do hazardous jobs. "What are they going to do if they are told to do other things? Say no and have a kiddie sit-down strike?" he asked.

Such jobs as lifting heavy bundles of laundry, he argued, could injure a teen-ager permanently. The changes also could have an adverse affect on the workers' performance at school and the extended working hours could result in teen-agers going home at dangerous hours, he said.

"I don't think the proposals will endanger anyone," countered Neville, who said many employers are turning away willing young workers because it is not "economically feasible to hire them when you know they are going home at 7 p.m."

The Labor Department's subminimum wage action would change the way employers obtain permission to pay less than the $3.35-an-hour minimum wage to full-time students. Employers now are required to get a waiver each year. The proposal would allow them to get certificates for more than one year.

The AFL-CIO called that proposal a "backdoor attempt by the Reagan administration to get a subminimum wage loophole which it couldn't win earlier in Congress."