PL. TRAVERS, the beloved creator of Mary Poppins, once told an admiring group of college students that the best way to write a book for children was not to write a children's book. That advice becomes even more valuable when the book is dealing with a cause, or an injustice, because in such a story condescension can be the ruin of fiction. Several recent novels,two for younger readers and four for adolescents, offer convincing evidence of Travers' advice.

The most successful is Cynthia Voigt's Jackaroo (Atheneum, $14.95; ages 12-up), a historical novel set in the days of lords, earls, and highwaymen. Young Gwyn, the innkeeper's daughter, must travel through both the personal and political pangs of adolescence. Unable to accept the injustice of the economic and political oppression which stifles her neighbors, Gwyn masquerades as the local legend, Jackaroo, a sort of mythical Robin Hood who, the stories say, materializes when there is trouble and avenges acts of injustice.

Although Gwyn successfully defends her people, her efforts isolate her from those she has helped. To avoid scandal, she must leave home, and find a new way in a very old world. It is to Voigt's credit that this search, and its conclusions, are both moving and believable. Both Gwyn, and Burl, the young servant who joins her in her quest, are strong -- determined to respect others, and one another. When they finally discover that they are in love as well, the reader is pleased to accept both their cause and their relationship.

Less convicing is another journey, that of Lissa Quintana-Green-Wolfe, heroine of George Zebrowski's The Stars Will Speak (Harper & Row, $11.49; ages 12-up). Zebrowski also offers his readers a journey through time -- and space -- as Lissa, in pursuit of her dream, leaves high school and her home on a distant man-made planet. She is convinced that she can help to translate an arcane intergalactic signal, and prove that there is other life in the galaxy. Though her ambition drives her to join the research project, she struggles with the conflict her love for a co- worker creates (but "it seemed too complicated, too roundabout, to feel love and pleasure, to be rewarded and denied, to dream of other things and then be stricken with needs that were just waiting to spoil everything").

The reader, however, is unlikely to be moved either by Lissa's commitment to her work, or the ease with which her conflicts are resolved. Zebrowski had a good idea, but his book is more a throw-back to the teen love stories of the '50s, too simplistic for the issues it tackles. And the last-page kiss of those '50s novels is replaced, in the '80s, by a clear though unexplicit sexual relationship, entered into midway through the book without mention of contraception or consequence. Perhaps such considerations won't be necessary in the far off future, but an author writing for "junior high and up," as the book jacket indicates, should know better.

NORMA KLEIN does better with sex, and almost everything else, in The Cheerleader (Knopf, $11.95; ages 12-up). Evan Siegal is a boy who, unlike many kids in contemporary young adult fiction, doesn't have too many problems. His stepfather and his mother love each other, though his father is something of a dud. His brother is a bit of a nerd, by Evan's standards, but has his own life and interests, and seems content -- especially after Evan intercedes to get him hooked up with the girl of his dreams. Best of all, Evan is blessed with a best friend worthy of his individuality, and a girlfriend, Laurie, who is cute, talented, and sensible about most things, including men. "Laurie thinks you should practically be engaged before anything really serious goes on. Mom doesn't have that much to worry about, unfortunately."

So with such a perfect set-up, what can go wrong? Plenty. Evan, almost accidentally, ends a classroom argument about the sexism of cheerleading by volunteering to be the first male cheerleader for the girls' softball team. He may have been fighting discrimination, but his success makes him the newest sex symbol at Haines High, and complications abound. Our 14-year-old hero, with the support of his family and friends, eventually untangles the complications any sex object must face. Everyone comes out a little happier, including the amused and delighted reader.

More serious problems daunt young Paris, hero of Rosa Guy's Paris, Pee Wee and Big Dog (Delacorte, $13.95; ages 9-12) Guy's challenge is considerable, for Paris and his friends Pee Wee and Big Dog live in a ghetto world where bullies carry weapons and hang out in burned out buildings, and mothers are often not home, even on Saturdays, "I'm sorry" his mother apologized. "I wouldn't have had to work if I hadn't taken that day off this week to go to court. But somebody had to . . . " Paris' mom has moved them downtown, to get away from the ghetto. She is determined to keep the slums at bay, even if she must sue the landlord to do it. The book is full of the derision and envious comments of old neighbors and friends about Moms' determination to live in a better neighborhood, a determination that drove Paris' father away.

There's a lot of reality floating through this gentle book, as Moms keeps fighting and Paris is almost led astray. But the pain of the city streets and of life for boys without fathers is touching and engrossing without being sentimental. It's not sociology, it's a story. Though all three boys are poor and black, it is the differences among them which intrigue us. For Paris, with his strong mother, there is redemption even in near disaster, but for Pee Wee, whose brother runs a gang and whose mom is never around, things don't look so bright. The counterpoint is Big Dog, younger, fat, with a pocket full of change and a father who takes him fishing. He adds poignance and perspective to this tale of one almost tragic day as the three friends encounter adventures Tom Sawyer never dreamed of. School librarians would do well to add this story to their shelves, even though its ending is pretty sad. City kids might find it familiar, and for the suburbs it offers a glimpse into a world far from that of Fourth Grade Nothings and Great Brains.

Also far from the Great Brain, in quality as well as substance, is Soup on Ice (Knopf, $9.95; ages 9-12), a mediocre book for younger readers by Robert Newton Peck. Though it would seem from his preface that he gets a lot of mail from his readers, it is hard from this book to figure out why. Though it too has a cause, it is an overmoralized tale of Christmas, mean saloon keepers with hearts of gold, hearty school nurses and widows' kids with no coal to keep them warm. It all sounds like a grown-up trying to sound like a kid, and though the behavior of the children in the book may constitute redeeming social value, so does Superman and he's more fun.

Fun too is the most overtly political of this collection, Mountain Light (Harper & Row, $11.49; ages 12-up), by Laurence Yep. This story is an unlikely success. It's about the early days of the Chinese rebellion against the Manchus, and the mass Chinese migration to America's gold mines when life became unbearable at home. Squeaky and his soon-to-be girlfriend Cassia encounter both the poverty of the peasants in China and the local squabbles which kept them subjugated so easily. Members of two different clans, they fall in love as they flee a failed rebellion in Canton. She is a serious revolutionary who has lost her father to the cause; he has always survived by being a clown. They learn a lot from each other as he gains enough self-respect to be serious and she learns to temper her anger with a laugh. When clan warfare forces Squeaky to flee to America, the reader is left with hope for him, for Cassia, and for his new country, whose hospitality is still so woefully incomplete.

All these writers deal with an issue in some way, and many of the ideas are idealistic, as young people should be encouraged to be. But as readers compare the six, the clear lesson, as Travers has always known, is that good intentions aren't enough.