BAGTHORPES ABROAD, by Helen Cresswell. Macmillan, $10.95; ages 10-up. Very late in this, the fifth entry in the (one hopes) neverending Bagthorpe Saga, Mr. Bagthrope reminds us, "We aren't here on a nonstop round of pleasure and idle self-indulgence." But they are, of course, all 15 of them -- if you include the family pets -- much to every reader's delight.

In this latest book, Mr. Bagthorpe's teleplay about the supernatural is rejected as unconvincing, and so he has his family installed in a haunted house in Wales. Research, he says. For the Bagthorpes, a wonderfully eccentric lot, it is merely another madcap opportunity which eventually involves even the local police.

This premise can't lead to sane goings-on (and wouldn't we be disappointed if it did?).

Helen Cresswell has an unlimited supply of wonderfully wacky characters to draw upon, so there's never a dull page. When events slow, there's always Mr. Bagthorpe's sarcasm. Oh, not that Mr. Bagthorpe is nasty. He's just . . . direct. When, for instance, one of the officers looks around Ty Cilion Duon, which translates as "The House of Black Corners," and says, "'er, -- nice place you've got here, sir," Bagthorpe responds, "It is not nice . . . and it is not mine."

Cresswell is so veddy British, too, in the way she describes events and characters, including furry ones. The dog, Zero, for instance, lounges about "in his customary boneless slump." Grandpa is said to have "the kind of face that you could warm to even if he had just run into the back of your car."

After you've read this book you'll undoubtedly use Bagthorpian as the author does: as a synonym for zany.

IN THE YEAR OF THE BOAR AND JACKIE ROBINSON, by Bette Bao Lord (Harper & Row, $9.95; ages 10-16). This charming book covers a year in the life of Bandit Wong, who is renamed Shirley Temple Wong for her journey to America.

It's wonderful to see the Brooklyn neighborhood where the family is to live through Shirley's eyes, for it informs us about her life in China, too: "All the streets looked alike. Each street was flat and painted black. There were no steep hills. Each house was a replica of theirs. Every place stuck to the next. Wall to wall, without any gardens. No moon gates or fan windows or stone lions. Now and then a tree, but no flower beds." It's wonderful to see Americans, too, and learn, for instance, that "Hair came in all colors in America."

In a wholly foreign culture, Shirley gets herself into some funny -- though not at all predictable -- jams. One of the first Caucasians that she meets, for example, has a pronounced tic. Shirley assumes it is part of the typical American greeting, and adopts it immediately and well. The result is that her teacher sends a note home suggesting that Shirley undergo an eye exam.

But readers will laugh with rather than at Shirley, particularly since she strikes chords that all kids find familiar. Regarding her parents, she asks, "How could grown-ups be so blind to the pain of those younger and shorter than they?" Even her rendition of the Pledge of Allegiance sounds remarkably like our own earliest recitals of those words: " 'I pledge a lesson to the frog of the United States of America, and to the wee puppet for witches' hands. One Asian, in the vestibule . . ."

We eventually see an acclimatized Shirley Temple Wong, awaiting the birth of her new baby brother (she is sure). She vows to teach him how "to chew gum and blow bubbles. How to smack a homer and walk a yo-yo and skate backward."

The book ends with Shirley meeting her hero, Jackie Robinson, who gives her "a grand slam of a smile." Thus she says of the time that the book encompasses: "This year of 1947. The Year of the Boar. The year when dreams came true. The year of double happiness."

THE COMPUTER NUT, by Betsy Byars (Viking Kestrel, $11.95; ages 10-16). We know right off the bat that it's Kate in the title role, entranced as she is by her father's computer. And no wonder: Kate receives a message which purports to be from Outer Space.

Kate's not easily fooled. She and her friend Linda assume that someone in their own school is behind it all. Willy Lomax, perhaps, for whom Kate just happens to have a mad but secret crush. But neither the mystery nor the crush comes closer to solution when Kate and her friend turn up at a dogwash to test Willy's reaction to hints about the eerie computer messages from BB-9.

BB-9 turns out to be a stand-up comic of a spaceman who is coming to Earth to try out a slew of new one-liners. Earth, he tells the kids, "is known as the Laughing Planet. . . the only planet in the universe where there is laughing."

BB-9's attempts -- despite the fact that he does a pretty good Steve Martin and a not-so-bad Don Rickles -- fall very flat. "Unfortunately," he says in parting, having narrowly avoided being beaten up at a local pep rally, "I was not as funny as I thought I would be."

Kids, however, will probably disagree.

THE ONE-EYED CAT, by Paula Fox (Bradbury Press, $11.95; ages 10-16). This is a poetically written novel about an 11-year old boy and the guilt which haunts him.

Ned Wallis' father is a minister; his mother, an invalid. Her brother, Ned's uncle Hilary, is a world-traveler, as expansive as his own folks are parochial. Hilary gives Ned an air rifle for his birthday, but Ned is forbidden by his father to use it.

One night Ned sneaks the gun out to the barn and fires at a movement in the night. He returns the gun to the attic and tries to resume his normal life, which includes doing chores for an ailing, aged man.

Then a one-eyed feral cat shows up, and Ned is convinced that, with that one shot, he, Ned, had caused its misfortune. Ned and the old man care for thcat, but meanwhile, events have turned as a result of Ned's act.

For one thing, caring for the cat necessitates some lies. For another, he cannot leave the cat to make a trip over school vacation with his uncle.

On the plus side, his efforts save the animal and cement a strong bond with the old man. When the man has a stroke, Ned is there to bring help, and when the old man is placed in a nursing home, Ned becomes his only visitor.

It is the old man to whom Ned confesses and it is the man's response which finally frees Ned to talk to his parents.

THE FRAGILE FLAG, by Jane Langton (Harper & Row, $11.95; ages 10-16). When Georgie Hall's folks and other grown-up protesters are unsuccessful at dissuading President Toby from his plan to park a nuclear "peace-keeping" missile in space, the 9-year old girl decides to take matters into her own hands. Armed with an old United States flag, Georgie sets off on foot, aiming to march from Concord, Mass., to Washington, D.C.

Needless to say, she is joined by countless kids, and their walk is dubbed "The Children's Crusade." The marchers are some 16,000 strong by the time they reach the White House, and their act is copied by children in other countries, too.

The result? "The President . . . understood at last. The country he governed was not a land of soldiers and sailors and fighter pilots. It was not a nation of Democrats and Republicans. It was a mighty swarm of children, eager to suck into their lungs the clear bright air, to yell and catcall and laugh and shout, to spring off the ground and come down hard, to grow to their full height. It was kids, millions of kids, budding and ripening, sprouting and blossoming, kids in their heyday, their springtime, their green prime. . ."

Thus Georgie changes the president's mind -- and the world's fate.