Q. "My neighbor has a little boy who just turned 5 (my son's best friend in kindergarten), and she always seems to be taking him to the pediatrician or the dentist for some checkup, even though he's obviously healthy.

"Now she's taking him to an eye doctor, although he sees just fine. That's her business, of course, but she says I should take my child too.

"He does get his regular doctor's visits and we've been to the dentist, but why should a child have his eyes checked when he can see perfectly well?"

A. The cheapest medicine a parent can buy is preventive medicine, and an eye checkup is a good example.

One child in 20 has some eye problem, such as the fairly common amblyopia -- lazy eye -- which can cause irreversible loss of vision if it isn't caught soon enough.

It happens if one eye is much stronger than the other and starts doing the work of both, making the weak eye worse and worse. The problem is hard to repair when the child is past 6 and almost impossible after 8, and yet it can be corrected in the early years by covering the good eye with a patch. This forces the weaker one to see as well as it can.

A child can have his condition discovered or ruled out easily but it may as well be done in the once-in-a-childhood visit to an ophthalmologist. Every child needs this visit so the eyes can be studied under drops to make sure that there are no other problems or eye diseases.

You can do a preliminary screen for lazy eye yourself with the Home Eye Test for Preschoolers, developed by the National Society for the Prevention of Blindness Inc.

The kit is a simple set of instructions, in either English or Spanish; a chart with four lines of E's -- each line smaller than the last -- that point north, south, east and west and a detachable card that has only one large E on it. A child is ready to be tested when he can point the arms of this E left, right, up and down and then point his own arm the same way.

To test him you will hang the chart on the wall, sit him 10 feet from it and have him hold a paper cup over one eye while you test the sight of the other one.

Every time you point to an E he aims his arm in the same direction and when he has answered all that he can he covers the over eye and does it all over again.

The child who can't read the E's on the third line, either with one eye or both, should go to an ophthalmologist or an optometrist.

To get the free test, write NSPB Home Eye Test, 19 Madison Ave., New York, N.Y. 10016.

Q. "A long time ago you wrote about sitters who cancel at the last minute.

"I have eight years of baby-sitting experience and I know: If sitters are hired more than one week in advance, the mother is setting herself up for this kind of behavior.

"When teen-age sitters are maniplated to sign away their weekends long in advance, they feel little guilt about breaking these engagements. It won't happen as often if the sitter knows these long-term commitments won't be regular, and if the overall attitude toward her is respectful."

A. It takes two people to manipulate. In this case one is the employer -- and the other is the sitter.

A teen-ager, like anyone else, can control her own life with a simple word "No." Any employer will respect the sitter who says she doesn't accept sits more than a week in advance, but if she takes sits in advance and regularly cancels she won't get any respect at all, or many jobs either.

Employers pay sitters to be reliable, not expedient.

Parents' Alert -- Getting good day-care information is almost as hard as getting good day-care.

Working parents should be interested in a "bibliography of bibliographies" compiled by Carolyn M. Aldrich of Camal Enterprises. She lists most places in the metropolitan area where a parent can get lists of agencies and sitters, from the Senior Citizens Employment and Services in Northern Virginia to the District's quarterly list of infant and day-care centers. Send $3 to P.O. Box 147, Fairfax Station, Va. 22039.