KATE HARRIS is 11 and three-quarters, and a nicer sixth grader you couldn't hope to meet. She has loving parents who run a real-estate office together; a funny, affectionate older brother, and a century-old great aunt with whom she has lively and enlightening talks. In fact, until Kate and her new friend Maudie Schmidt take part in a reading-enrichment program at Maudie's old elementary school, her only real probelm [besides self-consciousness about her size 9 1/2 feet] is her pre-adolescent obsession with "cool" behavior.

Maudie, on the other hand, is not cool. As Kate cheerfully explains, she's "one of those kids that everyone wants to avoid....I guess the reason in Maudie's case is that she's a little bit fat." Maudie is, however, determinded to become Kate's friend, and when the "in" crowd decides that Maudie isn't a pariah after all, Kate warms up. No one suffers for long in this charming, neatly contrived tale.

The going does get rough for a while, though. When Kate reads The Birthday Dog to a group of first graders, the book's gentle description of a dog giving birth provokes a riotous discussion of reproduction. Word of this impromptu sex-education class reaches the more conservative element in Kate's Massachusetts town, and chaos results. There is a furious exchange of letters in the local paper; Kate's parents' business suffers a setback; her teacher and the town librarian find themselves in hot water. A school board meeting is convened, with a lively debate led by "Parents United for Decency," most of whom have not bothered to read the offending book.

It is up to Kate, finally, to set them straight, and she does, because not only is she a decent, well-meaning child, but she has finally learned to think for herself. Miles handles the fraught issue of formal sex education intelligently and fairly; her concerns are freedom of discussion and clarity of thought, not politics. Nice children, nice families, a very nice book.

The benighted family in The Masquerade is another kettle of fish. Rebecca Walker is a highschool senior in Old Greenwich, Connecticut, a good student, cool and capable. She is also her father's favorite, a fact, it would seem, that is the mainstay of her detached existence. But then everyone in the Walker family is detached in one way or another. Rebecca's 16-year-old sister, Sarah, is a dancer who exercises compulsively and controls her every movement. Brother Eric, a Harvard medical student, is a raging hypochondriac who is cynical about everything except for his own imagined ailments. Little sister Eliza "seemed to Rebecca older than seven. Older than Sarah. She seemed intuitively to understand things the other children didn't know." Otherwise, Eliza is a cipher, and like the other characters in this disturbing novel, more pitiable than interesting.

Rebecca's parents are the real kickers in this crew. We first encounter Edward Walker on page 4 when he's hauled off to jail, where he decides to remain until he comes to trial for embezzling a fortune from his law firm's clients. Mother Alicia is vague, incompetent and neurotic. The veteran of one nervous breakdown, she is protected fiercely by her husband and children from even the slightest worry or frustration.

Quite a bunch-more like a drawn-out case history than a family. With Daddy cooling his heels in jail, the family's mansion on Long Island Sound is actioned off, and the four children and their mother move into a sordid two-bedroom apartment over a drugstore. The Walkers' economic decline is followed by a rapid emotional disintegration. Sarah runs away, Eric takes to his bed, Edward rejects his family and Alicia is hospitalized after a second, more spectacular breakdown.

Rebecca continues to hold the family together by taking a job in the drugstore and postponing her freshman year at Yale, but finally she begins to fall apart too. Abandoned and betrayed by her father, she begins a humiliating series of notquite-affairs with lower-middle-class louts from New Canaan. While Rebecca punishes herself for her parents' irresponsibility, her siblings-who had a more realistic grasp of the family's situation to begin with-shap up. Sarah returns, Eric stops malingering, Alicia pulls herself together, and even Edward begins to take an interest in his brood. At last, Rebecca comes to grips with her disillusionment and begins to acknowledge and grapple with her feelings.

At the end of this melodrama, the family is beginning to function again. And, improbably, no real harm seems to have come from their ordeal: Edward is about to be paroled and Rebecca has even managed to preserve her virginity. The Masquerade is affecting, even funny at times, but what a family! what a mess!