IT IS ALMOST impossible to be a parent in this country without eventually knowing about Judy Blume. She is the phenomenally successful author of over a dozen books that have found their way into the hands of a readership which, until Blume more or less pre-empted the field, had gone without a spokesman -- your children. Over 30 million of them have taken comfort from her words.

The characters in her stories are too fat, too unpopular, or too meanly treated by their best friends. They want to know about sex, with specifics, but are too embarrassed to ask. They live with parents who are getting divorced, favor their younger brother, or don't understand anything. Judy Blume does. She is one of those adults who forgot to forget what childhood is all about.

Now comes Blume with a book that flowed right out of her mailbox. Based upon the nearly 2,000 letters she receives every month from her mostly young readers, Letters to Judy: What Your Kids Wish They Could Tell You is an attempt, heartrendingly successful, to give parents a keyhole glimpse into the world their children inhabit. It is not a pretty place.

Take Molly, age 11. She comes to school one day and finds out that all her friends are ostentatiously crossing her name off their notebook covers. "It's Donna," (confides Molly to Blume) who has turned the class against her. They proceed to call her "Brace Face," "Tinsel Teeth," and "The Ratty Redhead." Molly's mother, sounding eerily like Everymother, tells her that "she's sick of this 'ganging up' thing, and would I please try to find some new friends."

Molly, attempting to confide in a teacher, is told to ignore the situation; it doesn't work. Weeks of social ostracism ensue. "I really need help. Last night I was in hysterics. I grabbed at my carpet and screamed very loudly. I thought of running away and also of commiting suicide . . . Then I thought of you and I knew you would understand. Please, please help me!"

We don't know what happened to Molly. As she does with all her correspondents, Blume answered her letter but none of her replies are included in the book. In fact, if there is any flaw in Letters to Judy, it is the fact that so many children are left floating in our minds, somewhere "out there" as they deal with situations, many of which are far worse than Molly's.

Blume hears from children whose stepfathers have sexually abused them, who are handicapped, have brothers in jail, are being torn by bloody divorces, and are struggling with the ramifications of living with retarded siblings. "Thank you for reading my note," writes Abigail, age 14. "I just hope this note won't bring you down."

Reading Letters to Judy with a slightly commercial eye, it is clear that Blume has a gold mine of new plots arriving in her mail on a daily basis. But far clearer is the urgent need of children to articulate their fears, anxieties and real problems. It is Blume's intention in this book to give parents a few hints as to how to ease their children's minds and hearts, based upon her own experience. Interspersed between the letters (divided into various categories) are snatches of autobiographical commentary from Blume's life as a child and parent which address various issues, such as divorce.

Twice-married and divorced, Blume admits that between marriages she was not honest. "At the time my children needed me most, I was least able to give to them. Even though I wanted the divorce it was a time of shock, hurt, anger, sadness and depression." She married almost immediately again "because I was terrified at the idea of being alone." Throughout Letters, Blume inserts her own trials and errors where appropriate, giving form and overview to what might otherwise have been a book too full of "cries and whispers" from anonymous children to endure.

It is not all woe. "Dear Judy," writes Melanie, age 11, "How does a girl kiss a boy -- arms around the neck or waist? Also, do you squeeze the lips real hard and are the girl's lips placed exactly on the boy's or is the girl's upper lip above the boy's upper lip or what?"

Whether the advice Blume gives to parents is advice you have already taken is a question only you can answer after reading the book. That reason for buying Letters is sufficient in itself. But the children Blume introduces are such radiantly candid and innocent human beings that we cannot help but look at our own children with a deeper understanding and compassion. Blume's correspondents quite effortlessly outshine the author which I rather suspect she is happy to allow. Letting them have center stage gives credit to the director. This is a generous book.