"Wood You?", a 1970 short story by Piers Anthony; originally published in The Magazine of Fantasy and Science Fiction, October 1970, available at the Internet Archive; reprinted in Anthony's 1985 collection Anthonology. A French translation, titled "Bon sang de bois", was published in Fiction #234, June 1973, the French edition of The Magazine of Fantasy and Science Fiction.
The story, as far as I remember, is about a child who is quite adept at wood splitting
At three and a half, Buddy discovered that he could split even the largest logs by hammering in a wedgelike scrap of metal until the wood strained and sundered. But his wedge was brittle and bent, and the hammer he used had a loose handle, so he had to be very careful. Not only did he have to study the grain, he had to analyze the general configuration of the segment, discover any natural cracks, and determine the general type of wood. There was quite a difference between soft, straight pine and hard, curved yellow birch! He also had to work around the knots, and sometimes to flake off outside sections along the circular growth-rings. But one way or another he could, in time, split any piece at all.
In fact, he was an expert wood-splitter by the time he achieved the independent age of four. His adult sisters had long since given up and let him play with his tools, for he could put up a respectable battle when balked. They had no comprehension of the intricacies of woodcraft and were forever and unreasonably scornful of what they called his tall stories about grains and types. "He's out of bounds!" they exclaimed, not knowing that all boys his age were out of bounds, but few were as specialized as Buddy.
and gets spotted by an alien and brought to an intergalactic wood splitting contest.
One day a free-lance field agent for the Snurptegian Confederation happened by, attracted by the measured tapping of loose-handled hammer on brittle, bent wedge. The creature ascertained that no adults were present (for they tended to be narrow-minded about extraterrestrials), approached the scene of activity, and waited politely while Buddy completed his incision. A final series of blows, a judicious poke with the jackknife, and the piece fell cleanly cloven.
—Bravo! the Snurp agent exclaimed.—A masterful job.
Buddy was taken aback. He hadn't noticed the visitor, and no one had ever complimented him on his talent before. "Gee," he said shyly.
—One is truly skilled at the art, the Snurp said.—What might one do with superior equipment?
Buddy looked at it. The Snurp had bug ears and worm eyes and slug feet, but was otherwise rather strange. Buddy did not understand all the words, but he liked the tone.
—How would one like to compete in the regional wood-splitting junior championship tournament?
The child goes quite far in the contest, facing opponents from other races, but has to quit when confronted with some strange alien type of wood.
**Final phase. Petrified Poplar, veneer grain.
The wood appeared. It was monstrous, a yard wide, and as hard as rock. Buddy found three suitable stress points, but they were impervious to his wedge points. It would take far more strength than he possessed to make headway there—and it looked as though three wedges would have to be pounded at once, to unlock this complex boulder.
[. . . .]
He tried his little hatched on it, hoping for the best. The blade rebounded from the surface, leaving only a scratch. He tried to swing the axe, but this was even worse. He had gotten nowhere, and time was passing.
—One must turn the—Snurp began.
**Disqualified! the inspector eye said immediately. **No advice permitted from the sideline during the phase.
I remember the story ending with the child being brought back to Earth by the alien who spotted him, a judge from the contest coming, fining the alien because he didn't have authorization to bring the child from his parents, and the judge making the child the winner because of his very young age.
The inspector eye appeared.
**This contestant places sixth, raw score, it said. **Award ratio now being calculated. What is contestant's maturity index?
—One must provide the information, the Snurp told Buddy.
"Can I go home now? I'm hungry."
—How mature is one? Of what physical/mental duration, relative to the adult of the species?
Buddy looked at the Snurp in perplexity. "What?"
"Oh, I'm four."
—That would be four sidereal revolutions of one's planet about its star, the Snurp said to the eye. —This species is mature at fifteen or twenty revolutions.
The metal eye focused on Buddy. **One quarter or one fifth of maturity? Standard for this tournament is one half. That would place contestant at par times two plus. First on index, despite failure on final phase.
—The winner! the Snurp cried joyously.
**However, contestant is beneath tournament age of consent. Provide evidence of parental permission.
—Conditions were too pressing to obtain—
**Conditions too pressing? Violation of regulations, Snurptegian agent. Your species has bad recruitment record.
—Unintentional! Oversight! Misunderstanding!
"Can I go home now?" All this talk reminded him too much of the bickering of his sisters.
**Immediately, the eye said grimly. **There will be a full investigation.
And suddenly Buddy was standing beside the chopping block behind his house, alone. That ** was certainly prompt!