Christopher Walken plays a small role as a gangster in the junky kids' comedy Kangaroo Jack, and every moment he's on the screen is a marvel of charisma over crap. As he did last year in ''The Country Bears,'' the cosmically uncategorizable actor (a possible Oscar nominee for his tender supporting performance in ''Catch Me if You Can'') provides brief oases of adult pleasure in a desert of shrill juvenile jokes and clanging chase sequences.
The Jack of the title (who, by the way, isn't a talker, contrary to the misleading TV commercials) is an Australian marsupial in temporary possession of a jacket containing $50,000, usually worn by Louis Fucci (Anthony Anderson), who's the best friend of Charlie Carbone (Jerry O'Connell), who's the gangster's hairdresser stepson from Brooklyn, sent Down Under to...like it matters. What does matter, to those fascinated by such trivia, is that this charmless, expense-deductible business trip to Australia was produced by Jerry Bruckheimer (now branching out into action junk for kids) and cowritten by Steve Bing (penning a lullaby for his son with Elizabeth Hurley?). Also, ''Kangaroo Jack'' costars Estella Warren as a wildlife conservationist who, in the interests of PG, swims in an opaque white tank top and flirts demurely with Charlie.