ARE YOU giving your children acting lessons? Why not? Don't you want to be supported in your old age by at least one movie star? Why else are you putting all that money and exasperation into having their little teeth straightened?

Truthfully, Miss Manners does not mean to encourage you to put your children on the stage, Mrs. Worthington. There is quite enough of that sort of thing going around. But it is wrong for a parent to allow a child to go out into the world unarmed with the skill of acting, as in "acting like a lady," "acting like a gentleman," or at a minimum, "acting like a human being."

This does not come naturally. Conveying an emotion from one's heart to one's audience is a technique that must be learned.

There are exceptions, the observation of which is what misleads parents into believing that children can perfectly well express their feelings and that a lack of satisfactory expression indicates that the feeling is also lacking. The child did not have to learn to make its feelings of hunger, pain or anger intelligible, they note, and so a failure to express pleasure can mean only one thing.

Indeed, that is not true. For every time that you observe a child's face light up on opening a coveted present, you can find 10 instances of its saying "Uh," with no expression, when given or offered something it wants. "Would you like to go to the game with me and pig out afterwards?" the parent inquires; "Uh," says the child; and the crestfallen parent puts it down to a lack of interest or enthusiasm.

That is not necessarily what it is. It is much more likely that the child was never taught to say, "Why, I'd love to!" in the proper tone of voice and with the appropriate accompanying expression.

This is the first and simplist acting job for which the child must be prepared: Projecting his actual feelings. If you are pleased, you must act pleased.

The second course in acting involves demonstrating general, basic feelings, rather than the less attractive immediate ones. If you don't actually like the present, you act pleased anyway, because you like the principle of being given presents, and hope to encourage the practice, because you may do better next time.

The third level is still further removed from the immediate feelings, because it involves acting out a wish to please others, on the off-chance that this will inspire them at some future time to try to please you. In other words, this is acting pleased when not actually being given a present.

Let us now try some acting exercises.

1. Acting pleased to see someone. (On level one, this means conveying actual pleasure in seeing someone; on level two, it means acting glad to see someone one likes but is not thrilled to see at this particular moment; on level three, it is acting pleased to see someone one doesn't like.) Expression: bright eyes. Dialogue: "How do you do?" "Hello," or "Hi," "How are you?" "Yes, school's fine this year," etc.

2. Acting grateful for a present or the offer of one. Expression: wide yes, huge smile with mouth dropped to indicate surprise. Dialogue: "I love it," "Oh, I'd love to," "Wow--that's terrific."

3. Acting out the idea that you would like to have a pleasure, such as a party invitation or even a visit that might lead to something more tangible, repeated. Expression: Eyes shining with reflection of happiness, mouth closed but curved in smile of satiation. Dialogue: "I had a wonderful time," "It was really good to see you," "I hope I'll see you again soon."

A child who can master all that will not only lead an enriched life, but will discover some way to be able to give his or her parents a luxurious old age. MISS MANNERS RESPONDS

Q. When should one leave a cocktail party if the invitation states that the party is from 6 to 8 p.m.? Am I correct in assuming that the host has a later engagement and truly wants the party to end at the appointed time?

At 8 p.m., I thank the host, get my coat (and gloves if appropriate) and leave. In every case, the party is still going strong. Have I been missing all of the fun, or have I been liberating myself from a bunch of inconsiderate moochers?

A. Possibly both. Six to 8 cocktail parties begin at 6:20 and end at 9:45. The only people (besides yourself) who take the numbers 6 and 8 literally are the hosts, who are thus led to believe, at 6 o'clock, that no one is ever going to arrive, and at 8 o'clock that no one is ever going to depart.

However, if they wanted people there from 6 to 8, why didn't they invite them for 5 to 7?

Q. Very few of my friends have a butler or even a maid these days to answer the telephone. If someone is ill, I hesitate to call them for fear they are sleeping. On the other hand, I feel guilty not to show interest in their health. What should one do under these circumstances?

A. Miss Manners has just the thing for you. It takes only a few minutes, costs only a few cents, and reassures your ill friend of your interest without adding to his discomfort.

It's called a letter. "Dear Jeremiah, I was sorry to hear you are ill, and I hope you will be feeling better soon. Call me if you get bored."

Now--isn't that nicer than what you have to say when you telephone, which is "You sound funny--are you getting enough rest?"