ONE OF THE MOST reasonable of childish protests is, "If you can do it, why can't I?" No sensible parent will admit that as a basis of argument. The best answer is "Because," and the next best is "Because I'm grown up and you're not," or "Because I'm the parent and you're the child," or "Because I'm bringing you up, you're not bringing me up."

A belief in the arguability of every premise is, Miss Manners assures you, a tremendous handicap in dealing with children. A belief that children and adults must be subject to identical rules is plain silly. If children are entitled to all the privileges of adulthood, why don't they take some of the responsibilities? Does your child ever think of paying the electric bill or reminding you that it's time for your annual checkup?

Do not suppose that you have trapped Miss Manners on that one and that you can now pad the expense account, sass your sister-in-law and chew with your mouth open, while forbidding your children to do anything of the sort. Indeed, fundamental morality and manners are not negotiable. Children will do as you do, not as you say.

Nevertheless, anyone who has gotten to the stage of parenthood has of necessity been navigating the murky waters of life for some time and acquired some sense of its shoals and currents. (Would someone please bail Miss Manners out of nautical similes that are closing over her head? Thank you.) The two basic excuses for doing something that you have forbidden your child ever to do are:

1. The situation is more complicated than you can understand or than I am in a position to explain to you. You'll have to believe that I'm using the best judgment I have to do the right thing, however it looks. (I know you're supposed to respect your parents, but I finally had to stop Grandpa from calling Mommy "that-woman-you-call-your-wife.")

2. My function here is to serve as a bad example to you. (I do believe smoking is bad, but I'm unfortunately addicted to it and I will not allow you to develop the same problem.)

Excuses are not needed once one has comfortably accepted the principle of the double standard for different generations, for doing things that are fine in adults but not in children, such as staying out late when one has to get up early the next day and not even saying what time one expects to get home, or even the great privilege of getting up and going to work the day after a fever, instead of giving it a day's rest.

The most delicate area is that of child rearing itself. Controlling someone else's life, bossing that person around, creating expectations and goals for him to achieve, serving, in other words, as both playwright and critic, is lots of fun. That's why people have children. You get into trouble trying to practice this sort of thing on a spouse, acquaintance or stranger. (Miss Manners saw an interesting example of the last only today. A frail old lady got onto the bus with some difficulty, only to be stopped by a hefty matron, obviously a total stranger, who said, "Now, you're too weak to be out like this. Don't you have a brother? You tell him he has to drive you in his car.")

Children understand the joy of this and are only too ready to issue instructions to one another. While not disputing the parent's right to correct them, they fail to see why they can't play, too. Why is setting a child straight a dutiful thing to do, while reciprocating brings on a snippy, "Don't talk back to me"? Because.

Actually, Miss Manners is not all that authoritarian about it. She believes in an occasional, "Yes, you're right; you caught me. Parents aren't always perfect either, and I shouldn't have done it." This is not out of fairness, but because it is such a thrill to the child, because not claiming to be perfect is itself a disarmingly ingenuous protection for the parent, and because it actually reinforces the standard. Everybody understands that what is left unsaid is " . . . but I am perfect most of the time, a demonstration of which is my graciousness in admitting fault." It even stores up a bit of sympathy against the inevitable day when the grown child has a true moral difference with the parent, and overruling is no longer possible.

Until then, Miss Manners will concede only one etiquette advantage to the child that the parent must not employ. It is never proper for a parent to use an innocent child as an excuse, as in: "No, I'm sorry, I can't help you fund-raise because Daniela gets upset when I leave her with a baby-sitter." A child, however, has blanket permission to refuse to do anything (except his duty) "because my parents won't let me."

.Q. I was recently in the nerve-racking situation of having to make a speech in public. The gentleman to my right on the platform, sensing my quiet distress, tried to comfort me by putting his hand directly on my thigh a number of times as he whispered reassuring Q. words.

It is my feeling that a touch is appropriate action to take under those circumstances, but that the hand -- a quick friendly squeeze -- or the shoulder are far more acceptable parts of the anatomy to touch than the thigh of a young woman.

If you agree with my position, please advise me how to express my discontent without appearing ungrateful to such well-intentioned gestures.

A. A speaker's platform is not the place to practice Miss Manners' usual solution to unacceptable advances, which is a scream followed by the apologetic statement, "Oh, I'm sorry, but you startled me." (The advance man, so to speak, always retreats then, claiming A. it was an accident, in preference to explaining the situation to those whose attention was attracted by the scream. He does not repeat the offense.)

Instead, lean over to him and say, with an apologetic smile indicating that you do believe he meant well, "I'm sorry but you're making me nervous."