Q. "Please let me know if there is any way to get my 3-year-old to eat vegetables and fruits. My boy has not had any (except French fries) for almost a year now. I have tried all ways possible but with no success. I am extremely worried and concerned about his food habits, although his doctor is not.

"Do you think he gets the necessary vitamins from the food he takes, like bread, milk, banana, cheese and fish? (He doesn't like meat or fowl.)

"If I give him fruit jucies like apple, apricot and pineapple, would it have the same nutritional value as eating these fruits?"

Q. "I am the mother of identical twin boys. At age 2 1/2 they are still devoted to their fruit juice from a bottle three times during the day (they use a cup for milk at mealtimes with no complaints).

"I hesitate to break the habit because it seems to be absolute heaven for them: they settle down, each with his favorite blanket, and dreamily carress their cheeks with the soft wool while drinking away, eyes at half-mast. It reminds me of someone leaning back with a fat cigar after a good meal.

"Should I be concerned with this lingering attachment? A part of me feels they should be allowed to continue until they are ready to stop. They are going through so many other changes right now -- they are really terribly 'two' -- that I hate to take away one of their props when they might need it."

A. In answer to the first question, the nutritional value of juice is the same as it is for the fruit itself, says D. J. Cecil Smith Jr., chief of USDA's Vitamins and Minerals Nutrition Laboratory at Beltsville.

The one thing it lacks, he says, is roughage (or "fiber" as it's called now), which helps the digestion, but a child can get this from other sources, like whole wheat bread.

Because juice is less filling, however, a child will drink more and take in more calories than he would if he ate a piece of fruit. So much sweetness also may make him lose his appetite for the proteins and carbohydrates he needs.

Lendon Smith, the doctor who wrote the popular Feed Your Kids Right (Dell, $4.95) has just published a sequel which should interest you. It's called Foods for Healthy Kids (McGraw-Hill, $10.95) and explains the physical and psychological effects that foods have on a child.

Now in answer to the question about those dear twins.

You may think of those bottles as their props, but they're your props too. To get a pair of 2 1/2-year-olds to be quiet at the same time is amazing; to give up those golden moments would be incomprehensible.

Q. We have a normal 3-year-old girl who just started a two-morning-a-week nursery school. She is in a class of 20 children and knows 5 of them well.

She's okay before she goes in the morning and doesn't cause a scene when she gets there, but she's weepy and tearful practically the whole time.

Her teacher believes our daughter is trying hard to be brave but seems to be genuinely sad when she's there. If true, should she stop until she's ready or should we persevere?

A. It isn't a matter of whether you should persevere, but if she should.

Ask her. If you hold her tight, listen hard, ask enough questions -- and wait for the answers -- you may find out she doesn't like to go so often or stay so long and you can cut back to once a week, or you can pick her up early.

You may find out that it's some specific thing makes her quake or you may not find out anything at all, but taking something from home may make her feel better. If she could take her teddy to school to watch over her from some high perch (which no child could reach) or keep a scrap of her blanket in her pocket, it might make all the difference.

She also might be more comfortable if she could play with some of the unfamiliar children on her own turf. Arrange for one to come home with her for lunch, but ask one of your daughter's old friends from school too. Three children usually don't play too well together, but in this case your little girl needs reinforcements.

If she's still sad, you might look for a smaller nursery school. Twenty is rather a lot of children to face at once. Or you might stop nursery school entirely for a few months. Three is generally a good age for it, but that's an average, and as every mother knows, her own child is never average.