"Kids usually get an earache around 5 o'clock Friday afternoon," announces a knowing father.

"That means you either find a pediatrician who will prescribe an antibiotic over the phone or you're in for a weekend of agonized howling. That's not as easy as it sounds either. You'd think you were asking for heroin . . ."

Says a mother, "I shopped around until I found a pediatrician who told me my son didn't need tubes. Nobody else had given him a hearing test . . ."

To tube or not to tube. One family's first child has had three tubes--no more waiting through a weekend for relief. Another child has done well without them--tests show no hearing loss.

The ear tube--a tiny plastic thing about two millimeters long that could pass for a Ken doll collar button--has become a mainstay for treating the pre-nursery school aching-ear set. And, perhaps more important, for treating some preschool ears that may not hurt, but are not hearing well. Pediatric otolaryngologists estimate that tubes are inserted in the ears of perhaps a million American children a year.

The idea of surgically slipping a tube into the eardrum of a child, usually under age 3, to permit a fluid-filled middle ear to drain, is perhaps 100 years old. The middle ear needs to be empty of everything except air to permit proper hearing. But it often fills with fluid or "gunk" after ear infections and other disorders cause blockages of the Eustachian tube through which air should pass.

In its earliest use in 19th-century Vienna, with no sterile techniques or antibiotics, the ear tube fell into disrepute and eventual oblivion until the early 1950s.

"Since then," says Dr. Kenneth M. Grundfast, chief of otolaryngology (ear, nose and throat) at Children's Hospital National Medical Center, "it has become pretty well accepted as a method of handling middle-ear fluid."

Today's operation, performed with a microscope under a general anesthetic and meticulously sterile conditions, is considered relatively minor surgery. Unlike even a tonsillectomy, it usually does not require an overnight hospital stay. It is still surgery, however, and many parents and pediatricians prefer to exhaust other medical alternatives first.

Grundfast, who does not like to use the term "last resort," believes that the operation becomes a "reasonable alternative" to parents and primary-care physicians after "frequent visits to the doctor for ear infections , endless antibiotics with their attendant gastrointestinal upsets, frequent daily battles to get the medicine into the child, concerns about side effects, about resistant organisms . . ."

Nor does there have to be an ear infection for middle-ear fluid--and attendant hearing impairment--to occur. Hearing impairment, Grundfast believes, may be an even more urgent indication for inserting tubes than repeated infections.

Although some studies indicate that by the time a child is 5, the middle-ear problem is outgrown, Grundfast believes that is not reason enough to avoid the operation, even if the hearing also has cleared up. Regarding the suggestion that even slight hearing loss can cause learning disabilities, Grundfast believes more extensive and better controlled studies are needed before this is confirmed.

Nevertheless, "Suppose," he says, "the hearing impairment is causing problems with socialization in the preschool child. Often a child who is not hearing properly in a preschool situation is not perceived immediately as hearing-impaired. He is often perceived as a child with a lack of attention, a lack of interest in a school situation or as a 'loner.' " (The child with fluid in one or both ears may be hearing sounds as though through an ocean.)

Says Grundfast, "If a classroom leader says 'okay, children, let's put our coats on and go outside,' but Johnny, sitting in a corner playing with blocks can't hear the instruction, the teacher gets the impression he just doesn't socialize well and doesn't follow instructions. This is a label that often sticks with a child . . . What a tragedy if that is due to persistent fluid in both ears. So suppose that at 8 years the condition has cleared up? What about those lost years between?"

A study of youngsters in a Seattle hospital, reported late last month in the Journal of the American Medical Association, has suggested that major risk factors for middle-ear effusion (fluid in the middle ear) include not only repeated ear infections, but chronic runny and congested noses, allergies and exposure to cigarette smoke in the home.

The study found that children with allergies and nasal congestion who were exposed to passive cigarette smoke were six times more likely to have middle-ear fluid than controls. The risk increased with the number of cigarettes smoked by their parents or other adults.

Researchers, doctors and parents alike will attest to the frequency of ear problems in the first five years of life. No one can say for sure, however, why some seem to escape the problem altogether--even, sometimes, with smoking parents. Still others with repeated infections suffer little from middle-ear fluid. Then there are some children with no symptoms at all, who have seriously impaired hearing because of the presence for months of middle-ear fluid no one discovered. Sometimes it is not found until a speech expert detects a lisp, or parents themselves notice a child's changed behavior or inattention and are alert enough to suspect a hearing problem.

A second study reported in the same journal, part of an ongoing analysis of 2,570 Boston-area children, suggests that the middle-ear disease problem is significantly more widespread than had been realized. According to the study, one out of three clinic visits resulted in a middle-ear diagnosis; 75 percent of the follow-up visits were for middle-ear problems and even 5 to 10 percent of "well-baby" visits turned up middle-ear disease. The study also found that socioeconomic status had little influence on incidence.

The findings "suggest that any intervention to decrease the incidence of [ear infections] or to hasten the resolution of [middle-ear fluid] would substantially reduce the costs of providing care to children and perhaps reduce the number of required practitioners."

Grundfast sometimes gets discouraged with primary-care physicians who "tend to have a black and white view of medical therapy so that they feel medicine is good and surgery is bad, and I think some of them convey this to their patients."

But he has great respect for parent intuition. "Parents are really the best detectors of a child having trouble. They pick up on a child not paying attention. When they bring this to the attention of some physicians and are told that 'everything is fine and just take more medicine and I'll tell you when I'm going to consider another alternative, maybe surgical,' well, that gives surgery a bad connotation.

"Then the parent worries about being overanxious. I think any worry is justified because parents know their own children.

"And I can tell you they are most often correct." Now Hear This

Ear-tube facts from Dr. Kenneth Grundfast, Children's Hospital National Medical Center:

* A child should be considered a candidate for insertion of tubes if he has had more than four separate ear infections in two consecutive seasons, or if he has had fluid in both ears for more than three months.

* Once inserted in a child's eardrum, an ear tube stays in place until it is extruded--in about 8 to 14 months--by the child's growing body. During periodic checkups, a physician will see that the tube has dropped into the ear canal or onto the eardrum and can be plucked out easily by the doctor.

* After the tube is out, the hole usually closes by itself, but in rare instances it does not. This causes no immediate problem and actually prevents subsequent buildup of fluid. Eventually, however, it may have to be closed surgically.

* A child with an ear tube and a head cold may have a discharge from the ear, which can be frightening to a parent. Ear drops usually clear it up quickly.