CHILDREN WHO are away from home for the summer generally want to take up letter writing -- because they miss their dear families, because they are anxious to share their new experiences and impressions, and because many camps have a policy of not giving them dinner until they have produced letters to home.

It is an art at which children are not notably proficient, Miss Manners has concluded after studying the most recent anthologies of Collected Thankyou Letters of American Children.

"Dear Aunt Lucy, Thank you for the book. It was neat. I got a lot of neat things. Well, I have to go now, Your friend, Joshua"

Somebody should tell Friend Joshua that while this may have gotten past his exhausted mother, there is no chance that Aunt Lucy will tie a satin ribbon around it and carefully place it in an ivory box against the day that he becomes a great man and she may dispose of it at an auction house to her material advantage.

Miss Manners considers that sufficient incentive for raising the state of the art. The letter from camp is an excellent learning opportunity, containing, as it does, so many of the emotional essentials of life.

The purpose of such letters is to complain, brag, beg and alarm. The desired effect on the parents who receive it is to reassure them of the mutually exclusive facts that the child is having a marvelous time and misses them terribly; also, a really skillful letter should produce tangible results, in the way of packages of sweets.

For example, serious complaints about the camp disturb the parents' satisfaction in having done the right thing, and should not be made unless the child finds the situation hopeless and wants to be fetched home. (Of course, by the time the frantic parents arrive, the child will have gotten a part in the play and life will look different.) But mild complaints, humorously phrased, instill a desire to pack off little compensations.

Although parents are the only people to whom one can brag undisguisedly, a prudent child might want to employ general social disguises for self-congratulation, thus taking time to indicate steady improvement on difficult tasks, rather than trumpeting only full-blown triumphs. It depends on how many times the child wants to be rewarded.

Begging is more effective when it is immersed in what appears to be sentiment. Alarms should be sounded only in the interest of suspense, and not, unless there is an emergency, to provoke action.

Here is a poor and, Miss Manners fears, common letter from camp:

"Dear Folks, They are making us write letters. Everybody is mean here, and I hate it. I can swim better than anyone else. Send me some candy and money, because what they have here for meals is garbage. We're going to raid the girls' cabin tomorrow night."

Compare Miss Manners' version:

"Dear Mommy and Daddy, I'm learning a lot here, but not how to identify what it is that they call food. I miss you. I'm working hard for the swim competition and I think I have a good chance. Most of the other kids have pocket money they buy candy and stuff with, but what I miss is homemade cookies. This afternoon I fell off a horse."

Now children, Miss Manners asks you: Which letter will get the better response? MISS MANNERS RESPONDS

Q. We all know that we repair or replace an item we have damaged in someone else's home. What do I do if the breaker does nothing?

A friend broke a valuable and favorite item and said he would make good on it. That was a month ago. I have since had the item repaired myself, as it was a favorite and frequently used. I know I cannot mention it or send him the bill.

I am beginning to resent this person, and I am considering ending the relationship because of this lack of concern for my things. But the person would never know why I was no longer friendly. What alternatives are open to me?

A. Miss Manners cannot recommend any action that seems to place possessions, however valued, over friendship. But she does believe that it might ease your friend's feelings if you sent him a note, reassuring him that you were able to have the item repaired and that you still value his friendship. Perhaps the juxtaposition of the words "repaired" and "value" will suggest something to him.

Q. Is it proper to give oneself a birthday party? How can one let people know it's his or her birthday, without being too obvious?

A. It is perfectly proper to say or write friends, "It's my birthday! I'm going to have a party! Please come to my party!" -- provided, of course, you are celebrating no higher than a 12th birthday.

After that, it gets stickier. A spouse or friend can give a birthday party in honor of a grown-up -- but not more than one to a lifetime, unless it is strictly for family -- or one can give a party and only tell people that it is one's birthday after they have arrived. This avoids the problem of their finding supposedly witty presents which are even more of a trial to receive than to give.

Q. My husband and I are newly married and we have a beautiful new home with a lovely guest book appropriately placed (we think) on the table in the foyer.

We have many guests coming through our portals. Do we have our guests sign it each time they come, or is it proper only the first time? When we have a mixed group, such as repeats and first-timers, how does one handle that? We hope you can solve our problem.

A. If your problem is that many guests keep coming repeatedly through your portals, Miss Manners can solve it for you. Just keep chasing them with the guest book every time, and you will soon have your beautiful new home to yourselves.

The institution of the guest book was one of the great plagues of the Victorian era, when people were supposed to compose witty verses for one another on a moment's notice. It is equally annoying, if less intellectually demanding, to be expected to produce original compliments on demand, and you will end up with a book full of scrawls saying "Had a fabulous time!" and a memory full of sour faces of people writing them.

Of course, if you just ask people to write their names, departing from your house will be no more of a nuisance than checking out of a motel. Miss Manners suggests you use the book on special occasions only, when you ask guests to pass it around so that you will have a momento.

Better yet, use the thing as a party diary, yourself jotting down the names and perhaps reactions of the guests for your own records, after the party. You will have a much more interesting book when it is done.

Q. A difference of opinion exists between my wife and myself as to proper placement on a dinner plate of the knife and fork, when one has consumed as much of the meal course as one intends to consume. We are agreed that orientation should be with utensil points at 10 o'clock and handles at 4 o'clock. However, one of us was taught to place utensils entirely on the plate to facilitate one-handed removal with minimal risk of inadvertent utensil dropping; the other taught to leave two to three inches of the utensils extending beyond the plate rim to facilitate removal of the utensils separate from removal of the dinner plate.

This hardly is a momentous problem, but perhaps you might offer your opinion in due time.

A. If this is not a momentous problem, Miss Manners would like to know what is. Nuclear proliferation?

Flatware is never correctly removed separately from the plate, and should therefore rest securely and entirely on the plate.

Q. A bride's mother telephoned an invitation two days prior to a long-planned wedding and reception. Other persons had received invitations mail some 10 days earlier. This phone call was the single contact made to me by this family in six to nine months. Previous commitments prevented me from attending this event. What are my social obligations in these circumstances? Was the bride's mother's telephoned invitation within socially acceptable guidelines?

A. Your only social obligation is to assume that the telephoned invitation was made bacause it was discovered that by some dreadful mistake, your written invitation had been misdirected.