SMALL AND innocent children make marvelously skilled extortionists, Miss Manners has observed. There seems to be a completely natural feeling that leads them, in a wonderfully open way with no promptingnecessary, to say such things as "Buy me that!" and "what did you bring me?"

Adults, operating under the disadvantage of having been more or less civilized, rarely have a comparable ability to reply with equally charming candor, "Don't be silly, dear."

This state of affairs is an example of why Miss Manners is always so puzzled to hear well-meaning people rattling on about not wanting to inhibit the naturalness of their children. Why not, pray?

Repressing the dear things, an activity of bygone days that was known as child-rearing, can only improve the state of the world.

Certainly Miss Manners cannot be alone in being uncharmed by unbridled greed. The child who is allowed to express this universal but nevertheless unattractive urge will end up the poorer.

A parent therefore has an obligation to the child, as well as to the household budget, to refuse all such demands. Even for occasions on which presents can reasonably be anticipated--birthdays, Christmas--a child should be encouraged to engage in hinting, rather than ordering. ("What a beautiful doll!" and "I wish I had a bicycle" are not much more subtle than "Gimme," but they are more charming.)

But Miss Manners recommends a strict policy of demonstrating that attempts to establish other present-giving occasions backfire.

The childish imagination, not to mention the childish legalistic inclination, is deft at proposing such events. Many parents have found themselves unwittingly accepting rules that they must bring presents back from trips, that a sick child is entitled to material compensation, or that any excursion, whether it is ostensibly for the purpose of sightseeing, going to the circus or visiting the doctor, is fundamentally a shopping expedition.

The clever parent counters this by showing that, left alone, he or she is subject to generous whims, but that petitioning drives all such impulses away. "Actually, I was thinking of getting you that to surprise you, but you've taken all the fun out of it, so never mind." Or "You're making me sorry I brought you--I thought the show would be a treat, but I wouldn't have bothered to take you out just to buy souvenirs."

The resourceful child will then turn to blackmailing grandparents or other likely suckers. Some of these can be enlisted to cooperate in the parental policy, but the only legitimate object of training is the child. All suspect loot should be rigidly analyzed ("How did Grandpa happen to get you that gun you've been talking about?") and treated like either diamond earrings offered to a respectable young lady by a gentleman of evil intentions (given back) or, if that would cause etiquette problems among the adults, like the door prize won by the chairman of the ball committee (given to the needy).

In sum, the child must learn that his own apparent surprise is a major ingredient of the pleasure and gratitude that encourage others to give you what you want. It is an appeal to base emotions, but isn't that just what children have abundantly available? MISS MANNERS RESPONDS

Q. I recently read in a local paper that a tragic fire occurred the night before a wedding took place, killing the bride's mother. The bride's dress and shoes were also destroyed.

The newspaper went on to state that the wedding was held on the following day with an attendance of 150 persons. A large reception was also held after the wedding and a hardy party prevailed late into the night.

My neighbor and I were discussing this, and in such a case, where there is such a close death in the family, what would one do? For me, it would be impossible to continue with wedding plans and a festive evening with my mother in a morgue.

I don't think this is so much a question of etiquette as it is of respect and compassion for the grieving father, if no one else. Miss Manners, what is your feeling on this?

A. Miss Manners' feeling is one of incredulity and outrage. How can any decent woman go through the mockery of a solemn ceremony when the dress and shoes on which she has no doubt lavished care, thought and affection, have been heartlessly destroyed?

Indeed, this is an etiquette question. Etiquette lays down firm rules for the decent ordering of life. If you leave matters to be decided by such unreliable feelings as respect and compassion, you may get--well, just look at what you got.

Miss Manners is not interested in the rationale of the bride, although she has no doubt that this lady has one. It probably has a lot of "Mother would have wanted us to go ahead . . ." on the surface, and some hard facts on money advanced to caterers and such underneath.

How the family felt about Mother is irrelevant, and how Mother might have felt--a tense usually lending itself to the selfish interests of the survivors--is even more so. The fact is that a crime has been committed against etiquette, and you are free to make other assumptions about persons who would do that.

Q. I recently acquired a son-in-law who came complete with a 15-year-old son from his first marriage. While my relationship with the son-in-law is just about cordial, I do have excellent rapport with said "grandson," whom I enjoy--that is, except for two areas.

First, his table manners are barbaric! It is as though he is not accustomed to using a fork (his roots are upper-middle-class). His father makes no effort to correct this situation, and the only time he eats in an acceptable manner, according to my daughter, is when I am at the table, having previously read him the riot act as to what I expect from him.

The same is true of his appearance--he lives in torn jeans, pullover T-shirt and dirty sneakers. His father brought him over to my home for holiday dinner in that garb. I explained the cliche' "When in Rome," he went home and changed--no scene, no problem.

Now--do I "take over" in the role as Keeper of the Manners, which only covers the infrequent visits, or do I read out my son-in-law and let the chips fall where they may? I should add, my daughter is not concerned where the chips fall.

A. You are wildly succeeding as a grandmother, against great odds, in exactly the manner Miss Manners prescribes for combining pleasure and duty in civilizing the young.

Why would you then consider turning into an unpleasant and impolite mother-in-law?

We all have a great obligation toward the children who come under our sphere of influence, which few people seem to understand; and we have an equal obligation to refrain from messing with the adults in our spheres, which even fewer people seem to understand.

The world is full of children whose parents have not bothered to teach them any manners, which you may perhaps have noticed. Occasionally, one of them will get lucky, and find a stepparent, adult friend, grandparent or other relative or teacher who will take over that task.

It is a more difficult task for such people than it is for the original parents, who get a fresh start and a presumption of authority.

Anyone stepping in later, with less clear authority, must first obtain the child's desire to please him or her. Affection, tact and a myriad of qualities that the child will admire and want to imitate are needed for this, in tremendous quantities.

But if a child observes a pleasantly ordered way of life different from that to which he has become accustomed, and if he is led into this by someone of whose love he is convinced, he will respond. Your grandson has already done this.

There is no way that you can be expected to perform the same feat with your son-in-law, or, for that matter, with your grown-up daughter. Your chance with her is past, and whether she decides to enforce manners in her household is now out of your control.

Perhaps she will see, from your relationship to her stepson, that manners do combine extremely well with love; perhaps not.

In any case, the boy is learning proper behavior from you, and will have it at his command in the future, whether or not he practices it now at home. It is Miss Manners' firm belief that children of bilingual behavior, so to speak, will choose the better when they are independent.

Feeling incorrect? Address your etiquette questions (in black or blue-black ink on white writing paper) to Miss Manners, in care of this newspaper.

Copyright (c) 1983, United Feature Syndicate Inc.