CHARLIE MALARKEY AND THE BELLY-BUTTON MACHINE by William Kennedy and Brendan Kennedy, illustrated by Glen Baxter (Atlantic Monthly: $10.95; 48 pp.; age 5-up).

Traditionally, fathers and sons have amused themselves with train sets or with afternoons in the ball park wearing matching visors. They still do those sorts of things, but the ‘80s have wrought more energetic and sensitive relationships, often beginning with Dad “helping” in the delivery room. Now they run in 10K races together, backpack in the Sierra and film ski movies in matching Vargnets. And, what shows they’re really tuned into each other, they write children’s books together!

The collaborators here are 16-year-old Brendan Kennedy and his father William, Pulitzer Prize-winning author of “Ironweed.” They insist this was a 50-50 enterprise, but that’s not as important as the story, which is pure fun in a malarkey sort of way. When Charlie (Malarkey, of course) discovers his belly button is missing AND his pal Iggy Gowalowicz loses his, too, they set off to find the dirty little things. They learn that Ben Bubie, a wretched man “with a face like a walnut,” buys and sells belly buttons, aided by a smelly, unpleasant-looking machine. So the boys plan a mission to put Bubie out of business once and for all.

As with kids at recess, the authors’ imaginations run silly with words (“moop, foofed, spoop”) and descriptions: The machine sounds like “fingernails scratching on a blackboard and a roomful of giants cracking their knuckles and six hundred and eighty-three chickens squawking. . . .” And its odor, well, you’ll see. Baxter’s 16 full-colored drawings have the Zap, Pow flavor of comics, a spiffy accompaniment for these comic heroes.