WELL-BEHAVED grown-up is a credit and a joy to his or her child. It distresses Miss A Manners to see so many children carelessly trampling on their grown-ups' tender feelings, when a little courtesy and attention would transform sullen and edgy adults into cheerful and cooperative parents.

The sensitive child will notice that grown-ups worry endlessly about the judgment of their peers, and can be thrown into agonies of embarrassment by trivial transgressions of conventionality. It does not take much effort to cater to these little prejudices, and the returns, emotional and otherwise, are enormous.

Adults are full of secrets. They find it humiliating to have people know the most basic, and often the most obvious, facts about themselves. A child is expected to reveal his age and grade level whenever asked, and he is constantly asked. But adult society pretends to make a mystery of such things as age and income, and the polite child will respect this, no matter what he thinks of it. He will steadfastly claim ignorance of what birthday a parent has celebrated, what was paid for the house, and the dinner table discussion of who is getting ahead of whom at the office.

Loyalty also demands that the child suppress any knowledge of behavior that makes his parents look bad. Cute stories about their squabbles and their party behavior should not be repeated outside of the family, no matter how amusing.

On the contrary, the child has an obligation to defend his family against any disparagement, no matter how justified he believes it to be. To know, better than anyone, how impossible your relatives are, and to be ready to kill the outsider who suggests it, is the essence of family chivalry.

Respect must be shown to the grown-ups' peculiar ideas of what is proper and what improper in matters of dress and behavior. The intelligent child knows that such customs are not questions of right and wrong but of transitory group standards, and he is able to master more than one standard. To dress as your parents prefer when going out with them or appearing before their friends should entitle one to adopt the fashions of one's contemporaries when one is among them.

Observing the parents' social rituals, including pretending to be interested in their friends, is extremely important. The child who greets his parents' guests, engages in small talk with them, and excuses himself politely when he can bear no more of their silliness, has earned the right to have his own friends made appropriately welcome.

None of this is difficult to perform, although it requires some patience to make parents understand that courtesy is reciprocal. In the end, a parent who is continuously exposed to high standards of politeness and consideration cannot fail to be the better for it. MISS MANNERS RESPONDS

Q. What is the most effective way to apply for a raise? I don't want to be impolite and brag about myself, but the truth is that I deserve the raise and am not going to get it unless I ask for it pretty forcefully.

A. The most effective way is to mention politely that your employer's chief competitor has just offered you a job at twice the salary. There is, however, some risk involved in this method. One, he might congratulate his competitor the next time he sees him, and two, he might congratulate you on the spot.

Therefore, Miss Manners advises the more subtle method of using your own authority to establish that you are worth more than you are getting. Do not fear you are being impolite. What is called bragging in the social world is called self-confidence in the business world. Length of service, devotion to the company and faithfulness in performing one's duty are valid reasons for having one's salary gradually increased, but these dogged virtues rarely inspire employers to provide rewards.

If you add to them proof of imagination and energy in the performance of your tasks, you are more apt to suggest that you would also have the imagination and energy to place yourself better if you were not satisfied. In the working world, as in teen-aged romance, the person you treat best is the one you might lose, not the one whose feelings would be most hurt.

Please note that Miss Manners believes in citing worth, rather than need, as the reason. The reason that $100,000-a-year executives with paid-off houses and grown-up children are worth the money to their companies is that they all possess the skill of proving to junior employes that the latter are not nearly as needy as they plead that they are.

Q. I am uncertain about the correct way to eat fruit, although there's nothing I like better. Does it depend on what type of fruit is to be eaten, or the circumstances under which it is served or is it merely a matter of individual preference? I am referring to such questions as peeled vs. unpeeled, hand vs. fork, etc. More and more often, probably because of everybody dieting, I see bowls of fruit used as centerpieces at dinner parties, later being passed as the dessert course.

A. Fruit occupies the place in the food world that the ingenue does in society. That is, it is usually fresh (but occasionally stewed) and, although welcome anywhere for its charm and simplicity, it requires more complicated treatment when going about socially than it does when it is just hanging around the house.

Also, it is a mistake to treat individual members of this group in the same fashion, because some of the strongest looking ones are frank and adaptable, while others look soft but have hearts of stone.

The least formal way to eat fruit is to pick it from a tree or fruit stand and pop it into your mouth with your hand. It is customary to secure the permission of the owner, and a good rinsing doesn't hurt, either.

The most formal method is to attack it with fruit knife and fork until it is halved, then quartered and, if you wish, skinned. As all this fierceness might be considered overkill with, say, a grape, small fruits are formally eaten by hand, with the pits quietly transferred from tongue to hand to plate.

Here are the details:

Apples--Informal: Grasp at the ends, bite and rotate. Formal: Stab with fork, quarter and peel (optional); eat with fork.

Apricots--Informal: Same as apple, but with emergency napkin poised; rotate from left to right, as well as back to front. Formal: Halve, cut out pit, and eat with fork.

Bananas--Informal: Strip gradually, using bottom part as holder. Formal: Strip peel entirely away, cut slices and eat with fork. N.B.: Eating a banana with a knife and fork is almost as funny to spectators as slipping on a banana peel.

Berries--Informal: Grasp stem with hand and pull while securing berry with teeth. Formal: Use a spoon.

Cherries--Informal: Same as berries, then remove pit with hand. Formal: Use a spoon, especially if the cherry is nestling in ice cream.

Grapes--Informal: Grape goes in by hand, and seed comes out by hand. Formal: Cut cluster with grape scissors and then follow same procedure as above.

Oranges--Informal: Cut in quarters and hold by peel while slurping noisily. Better yet, squeeze into glass. Formal: Peel and then cut, or cut and then peel, whichever is easier (both are nearly impossible) and eat sections with fork.

Peaches--Informal: Same as apricots, but with more absorbent napkin. Formal: Cut in quarters around pit, peel and eat with fork. Apologize to hostess for staining tablecloth.

Pears--Informal: Same as apples. Formal: Fresh, same as apple; stewed, with dessert spoon and fork.

Pineapple--Informal: Same as formal. It is a mistake to hold unpeeled pineapple in the hand and bite into it. Formal: Quarter, cut from peel, slice and eat with fork. Wonder why hosts didn't perform this in pantry.

Watermelon--Informal: Same as pineapple, but flicking away seeds with knife. Highly Informal: Put face into watermelon and see who can spit the seeds farthest.