A boy and his (magical mechanical toy) dog

Xu Jiao (left) plays the son of Stephen Chow, who also wrote and directed the film. (Sony Pictures Classics)

Whenever some news show freaks out over a child prodigy, the kid usually has one trick. Admittedly, it's always impressive: He's been composing full-length operas since he was 3; she hit the world's fastest serve in tennis at 8; he can hack into your bank account between diaper changes. Bravo, boy and girls.

But frankly, Rachmaninoff at 5 is easy. Comedy at 9 is hard. But Xu Jiao might already be a virtuoso. Her exuberance sprays all over Stephen Chow's magic-toy farce "CJ7" like a can of agitated Jolt. She has impeccable slapstick timing. (Witness a hilarious scene with her, a discarded banana, and a Dumpster.) She makes dozens of faces, uses a thousand vocal inflections, and does both with unusually natural intelligence.

Mind you, she's managing all this while playing a 9-year-old boy named Dicky. Xu is so convincing that I didn't know what gender she was until five minutes ago. (Cate Blanchett, eat your heart out.) And Xu's never even acted before! If more American parents took their kids to Hong Kong-style family-friendly sci-fi (subtitles, shmubtitles), Xu would be a star.

"CJ7" is precisely the 80-something minutes of delirium and cheesy special-effects you'd expect from the man responsible for the chaos of "Shaolin Soccer" and the lunacy of "Kung Fu Hustle." Cheap is how Chow rolls. But the movie's technical shabbiness is very much its point. When Dicky and his single dad, Ti (Chow), smash cockroaches on the wall of their dingy janitor's closet of an apartment, it's a blast for them.

Ti works a grueling construction job to keep Dicky in a fancy private academy, where the rich kids howl at him for being a goodly idealist. They hoot at his poverty, too. One night Ti comes home with a ball he found in the trash. The ball turns itself into a fuzzy version of the mechanical toy dog Dicky's classmates are going nuts over. Dicky's model, a CJ7, is magic. Astonishment flashes across his filthy face. After he bounces around to the Boogie Pimps disco version of "Sunny," Dicky decides to use his new pet to attack a vicious dog and help him ace a quiz. He wants to use it to get rich.

When that doesn't quite work out, Dicky gets testy, and he and his fuzz ball fall into a hilarious love-hate relationship. Boy curses magic-dog ("You're low-tech and boring!"). Dog poops on boy. But Chow isn't the sort of director to find any of that perverse. This is all of a piece with his usual hard-working charisma. The actors, especially the very talented kids, sweat from just cracking a smile. "CJ7" is basically Chow's exclamatory tribute to "E.T." - Spielberg by way of Nintendo. The music has the occasional John Williams flare-up. And the plot relies on Dicky's pet to be a sentimental lifesaver, too. But Xu Jiao's Dicky is the sweetest, most surreal homage of all: He's Gertie and Elliott.

Wesley Morris can be reached at wmorris@globe.com. For more on movies, go to boston.com/ae/movies/blog.