Q: I am a divorced mother with three children: a 10-year-old, 9-year-old and a baby of 14 months.

My problem is with my baby sitter, a neighbor's daughter who is just 11--only a year older than my eldest (who resents this because she is in charge when they go to their father's). The sitter doesn't know how to change the baby's diaper or prepare his bottle. But the biggest problem is that she has friends over who are noisy and messy. When I get home, there is usually a sinkful of dirty dishes and candy wrappers are all over the house.

My 10-year-old says this girl makes them go to sleep early or stay in their rooms, and then they hear loud music and giggling downstairs. A couple times when I've returned, the house smelled funny and was smoky. When I asked the sitter about this, she said she was burning incense.

What should I do? I've thought about getting a different baby sitter, but most of them charge much more. This one is only 75 cents an hour ($1 after midnight).

A: Are you sure your sitter shouldn't be paying you? It sounds like your oldest child does the work while the sitter gets a free place to give parties.

This is the world turned upside down. A sitter is supposed to sit with the children--to read to them, play games with them and talk with them as if they were real, live people, not puppies to be shooed away. And a sitter is supposed to be old enough and smart enough--and interested enough--to know how to prepare a bottle and change a baby before she ever takes a sitting job, or at least to learn how by the end of her first day.

She also should clean up any mess she makes and leave the house as it looked when she got there--no better, but certainly no worse.

And she should not have company, ever. It doesn't matter whether she is paid 75 cents an hour or $75; an employe doesn't take visitors to work. And she surely doesn't burn incense to hide the smell of other things they might be burning, like cigarettes--or pot.

This is an unhealthy situation; you must know that.

A sitter, after all, is your "surrogate"--the person you hire to react as you would. Do you really think this child would be capable? Would she hang up on an obscene caller? Refuse to let strangers in the house? Would she be able to handle a grease fire on the stove? Could she evacuate the house if the flames got out of hand? What if the baby had a convulsion or took a bad tumble?

Those are scary possibilities, but the thought of them should make you realize why you need to hire older, more sensible sitters to handle unanticipated problems. It isn't fair to give any sitter responsibility before she's able to handle it. And it particularly isn't fair--or wise--to give this much responsibility to the sitter you have.

You can dismiss her simply by changing sitters, without any explanation, but it would be more straight forward to invite her over for a quiet discussion--just the two of you--and tell her why you are displeased. She has the right to know what a sitter is supposed to do--or at least realize the possible consequences of her actions--so she won't make the same mistakes when she finally is old enough to sit.

This young person is behaving at your house in ways that, one hopes, she wouldn't be permitted to at home. If something bad happened, her parents might argue that you were contributing to the delinquency of a minor by not setting higher work standards for their daughter, or paying attention to possible danger signs.

You also might feel a responsibility to the child's mother--as one parent to another--and tell her about the parties and the incense. She might realize that her daughter needs a sitter of her own.

An 11-year-old is not old enough to sit at night. Preteens can be great sitters, but for only an hour or two a day, to walk a bored baby, for example, or to fill in if a parent is sick. Usually a child should be at least 13 to be a good sitter, but age is still no guarantee.

Even though your sitter charges little, she is expensive. You're worried, your eldest is angry and your children are getting short shrift.

Older sitters do cost more, but perhaps you could pay them in time instead of money by finding one or two other mothers--through church bulletins, community bulletin boards, ads in your area paper--with whom you could exchange hours in each other's houses. You might even work out a baby-sitting cooperative--which is like money in the bank--and pay each other in time vouchers. This arrangement has been the resource, and the salvation, of countless families.

And if you still think you can't figure out a better solution, look at your children. They are the three best reasons you have.

Questions may be sent to Style Plus, Parents' Almanac, The Washington Post. Worth Noting

Family therapist Margaret Mudd and family consultant Barbara Hitchcock will conduct a Sunday-afternoon, three-part "experiential workshop" for families March 6, 13 and 27 at American University. The "Appreciating Family Diversity" sessions, from 1:30 to 5:30 p.m., are designed for families with children age 10 and over. Cost for the entire family, all three sessions, is $75. To register, call 686-2500.