THERE IS nothing wrong with children playing with their names, provided that they clean up the mess when they are finished. Miss Manners even recommends that parents give their children the proper equipment - middle names, good combinations of initials, nicknames, names which have alternate spellings - to use when the children inevitably decide that they can no longer tolerate their childhood identities.

What Miss Manners objects to is grown-up people who continue to play with their names, and then are insulted when their friends can't keep up with the changes.

Changing names around has always been an American habit, but it's getting worse. Take, for example, a typical American family of four generations.

The family's American history began with a man who Americanized his surname when he arrived, and a woman those last name reflects a mis-spelling by an immigration officer.

In the next generation, there is a daughter who married three times, changing her name successively to the full name of each husband, preceded only by "Mrs."; and a son who is a movie star and was issued a completely new name when he signed a studio contract.

After that, there is a daughter who has a hyphenated last name, consisting of her maiden name and her husband's last name. She is no longer married to him and has, in fact, married someone else, but she must retain that hyphenated name because she has made her professional reputation with it. Also in this generation is a son who has had an attack of ethnic nostalgia, and changed his name to that of the family name in the old country.

These people's children, the result of unions with various people who had changed their names to X, Ben Moses and Khrishna, are called Che, Noah, Vishnu and Kunta Kinte. They are still young, but they are already plotting to change their names because they have found that all the other children in their respective home rooms are named Che, Noah, Vishnu and Kunta Kinte.

Miss Manners had decided, for the sake of order, to put some limits on all his. Here are the new rules:

Up to the age of 17, children are allowed free play with their names. They are even allowed to change the names which end in "y" to "i" and vice versa, which of course they all do anyway.

Upon leaving high school, they must each pick a permanent first name. On beginning college or employment, they may tell everyone the new name, pretending they have always had it, but they are not allowed to chastise relations and childhood friends for using the old one.

When they either marry for the first time or settle on a first serious career, they must pick a permanent last name. It is wise not to associate these names with philosophies or spouses who are likely to prove fleeting, because this is the surname they must keep.

Miss Manners suggests sticking to the original family surname - but in the female line. The basic family unit has now become the mother and children of whom she has been awarded custody, and it is simpler if they all have the same name and keep it, no matter who happens to join them later. The system of the matriarchal line worked fairly well in ancient societies, before women made the mistake of telling men that they had any connection with the production of children. MISS MANNERS RESPONDS

Q: Would you comment on the decline of elegance. The fashions today look like rejects from a garage sale.

A: Elegance in fashion didn't decline: It should short the day Worth stopped providing wardrobes to heroines of Henry James novels, among others, who traveled seasonally to Paris for the purpose.

Q: I am about to go into retirement and wish to stop sending gifts, due to insufficient retirement income. I am a godmother to two girls (same family) and wish to stop sending birthday and Christmas gifts after both girls pass age 21. How do I not hurt the older girl's feelings by continuing to send the younger girl gifts but not any to her?

A: By explaining the situation to her. If her feelings are hurt by the fact that your income is insufficient, then this godmother has neglected her girl's spiritual education.

Q: For business letters when it is known that the recipient will be a woman. "Dear Sir" is clearly wrong, as is "To Whom It May Concern." In addition, the term "Gentlemen" does not take female recipients into consideration. What do you suggest?

A: In business correspondence, Miss Manners uses the term "Mesdames," assuming that any males in the organization will understand that they are meant to be included. If Miss Manners is addressed as "Dear Customer" in commercial correspondence, she replies to "Dear Merchant."