No diagnosis frightens a parent more than "failure to thrive" -- that unaccountable condition that puts a newborn life in peril.

Sometimes older children don't thrive either -- emotionally or mentally -- and this dismays a parent almost as much. And it should. Squabbles, boredom and a little rebellion are normal in children, but when you have a child who can't sit still for five minutes, can't concentrate, can't learn, can't think straight, a parent knows there is trouble ahead.

Two recent columns -- one on hyperactivity and one on learning disabilities -- brought poignant accounts from parents who stumbled through a forest of answers before they found the right ones for them.

Q. "Your column on hyperactivity brought back 21 years of concern for my daughter.

"As a child she was allergic, had the proper shots and was relieved of her problem, but she cried a lot and was hyperactive. We were told not to be overly concerned and that she would outgrow her problems.

"We would ask her teachers why she couldn't pass multiple choice questions or work math problems accurately when her daily grades were above average but we got no answers. Her Stanford-Binet I. Q. score went from 131 in grade school to 101 by the time she was a high school senior.

"We spent thousands of dollars, visited every type of doctor we could think of -- psychologist, psychiatrist, optometrist -- and we even tried the Feingold diet last year, but it was too hard for our daughter to follow in college.

"Finally we got our answer in a $35 examination after I read about the problem in the paper.

"We went to an optometrist who is a vision specialist and found that our daughter had one eye higher than the other; her contact lenses did not correct her astigmatism; her near-to-far focusing was very poor and her eyes were competing with each other, causing stress, increased concentration efforts and hyperactivity.

"She needed a program of vision therapy and new lenses to correct these problems.

"Now why did I have to be the one to find out the problem? Why isn't this test part of the school eye test?"

A. Why did it take a parent to discover the solution? Because only a parent would keep looking for 21 years -- and for all the money and effort, you know it was worth it.

A problem like hyperactivity can have so many causes it often means traipsing from expert to expert, only to find the diagnosis is likely to fit his favorite theory. A specialist may be willing to stop there, but a parent knows you don't give up until you either find out what's the matter or the problem goes away.

Eye exercises, while not the universal answer, can correct hyperactivity for many people -- even adults -- who get the heebie-jeebies because words and letters and numbers intermingle on the page.

In fact, eye exercise are required of all school children in China every day, in an effort to prevent and correct eye problems, according to Dr. Morton Davis, an optometrist and one of the area's few vision specialists.

To see if a child might have a vision problem, he suggests that the child hold a tube of paper towel -- or a rolled up sheet of paper -- to his right eye, then hold out his left arm with the palm upright. It should rest alongside the end of the tube without covering the hole. The child then looks with both eyes open to see if he can merge the two images -- the hand and the hole. If there's no vision problem, the hole should move to the center of the hand and this also should happen when the test is switched to the other eye.

If parents and teachers want to know more about it, Dr. Davis and his colleagues will demonstrate these exercises free at their offices, 4905 W. Cedar Lane, Bethesda, from 8-10 p.m., Jan. 26.

Q. "Our 6-year-old daughter was diagnosed as having sensory integration dysfunction when she was 4 and now she has just about every area of disability you mentioned. Luckily she was caught early and is receiving intensive help in her Prince George's County public school. It wasn't easy getting the services she needed. It is the parents' responsibility to see that their child's needs are met and the parents' right as a taxpayer to demand it."

Q. "Some learning problems and disabilities may be preventable.

"Our daughter, now a fairly comfortable fifth grader, struggled unhappily through her early school years which I believe could have been prevented if she had started school a year later.

"Because she is bright, outgoing, artistic and creative, I tended to downplay the significance of her speech difficulties, short attention span, problems in following directions and slower development of coordination. She was labeled 'immature,' but in observing the sudden and spontaneous way she matured in various areas of development, I believe that her own genetic 'developmental clock' was set on a different schedule.

"I found this idea in a book called 'Better Late Than Early' by Raymond S. Moore and Dorothy N. Moore (Reader's Digest; $3.95). I only wish I had read it years before. It convinced me that we should introduce reading and formal learning with much more caution."

A. The Moores aren't the only ones who question early school entrance.

The Waldorf School, a small private school in Washington, is founded on the belief that the body, the mind and the spirit are integrated and that the child must grow in harmony both with nature and himself. That's why they don't put a child into first grade until he loses his first tooth -- a sign of general readiness.

This elementary school, which follows the Rudolf Steiner method of education, combines the subjects too, using arts and crafts, gymnastics, storytelling -- and a lot of repetition -- to teach academic skills. Although the classes follow a precise schedule, with a theme for every day and every year, a child still learns at his own pace. Reading is expected when it happens, which creates a gentle atmosphere that is just right for some children.