In a bid for more passengers during the slow winter months, Trans World Airlines yesterday proposed a variety of promotional fares including one that would allow two children under 12 to travel free with their parents.

The kids would get seats, meals and everything else the paying customers get.

D.M. Casey, TWA's senior vice president for marketing, said yesterday TWA would ask the Civil Aeronautics Board for permission to institute the new travel plan for all travel during February and March on any of the airline's domestic routes.

The proposal would allow one child under 12 to fly free when accompanied by an adult traveling on a coach or discount fare, but not on a first-class ticket.

TWA also will ask the CAB permission to institute a new fare plan that would allow a passenger buying a full-fare coach ticket to take along one other person at half the full-fare price. There would be no advance purchase required for this ticket plan but roundtrip travel would have to be completed within seven days and the travelers would have to stay over a Saturday night.

Along with the new fare plans, TWA is asking to liberalize the Super Saver discount fare by reducing the advance purchase requirement from 30 to 14 days and by substituting the seven-day minimum stay requirement with a provision requiring the ticket holder just to stay over a Saturday night.

Casey pointed out that a family of four traveling from New York to Los Angeles would be able to make the roundtrip for as low as $450 during February and March by combining the "Kids for Free" with the Super Saver fare.

TWA officials also disclosed that the airline is abandoning the three classes of service plan it instituted in mid-October that put full-fare coach customers -- with a lot of fan-fare -- in their own section with the promise of more for their money.

The plan, designed to appeal to the business traveler, apparently failed to appeal to either the business traveler or the discount passengers, according to TWA spokesmen. The full-fare passengers who expected to find empty seats and more roominess, found that their sections were crowded because of heavy air travel and didn't perceive that they were getting anything extra. And the discount passengers knew their services were being cut back. On top of that, United Airlines, a major TWA competitor, didn't match the service and announced instead it was going to upgrade service for all.

TWA said it hopes to have the three separate classes of service phased out by Dec. 6 and will go back to the traditional first class and coach sections.