Donny Osmond, flag-waving and theatrical history are the three attractions of "Little Johnny Jones," at the Kennedy Center -- plus a creditable portion of a luxury liner that lights up at night and pulls away from shore. Goodies, all, but not necessarily enough to cover the time and money one spends at a full-scale musical comedy. This is the 1904 musical written by and for George M. Cohan, with Osmond making his stage debut in the title role. Fans from the 24-year-old song-and-dance man's 19-year career will be happy to see him, but even people who are still wincing from hearing Osmond intone "Go, Ronnie, Go" at the inaugural gala will find his bouncy tap dancing and cheery voice attractive on stage. (In the modern custom of musical comedy, his voice is miked, although his teeth project throughout the theater.) There is also a lot of good, noisy enthusiasm from Maureen Brennan, as the heroine, Goldie Gates; Anna McNeely, as her aunt; and Jane Galloway as Florabelle Fly, the society editor of the San Francisco Searcher. The characters' names tell you a great deal about the humor of the show, particularly when you add a Chinese named Sing-Song. Theatrically, "Little Johnny Jones" was a milestone on the route from vaudeville to the musical comedy. It has two still-popular songs, "Give My Regards to Broadway" and "Yankee Doodle Boy," which are fun to see in their original context, and an obscure satire of a Gilbert and Sullivan number, which is mildly amusing. (The rest of the songs are awful.) It should be remembered, however, that pleasant but mindless entertainment was more welcome on the stage before television and movies went into that business. What holds this show together is certainly not the story line about an American jockey out to win fame and love in London, but a spirited patriotism of the kind that is generally called "unabashed." This being one of those intervals when "good clean fun" is popular on Broadway, the other kind of fun having been so thoroughly exploited in recent years, it has its charm. But it's a funny kind of defensive patriotism, in which American simplicity is extolled as the opposite of European snottiness, a catch-all that includes culture as well as aristrocracy. (For a more elegant version of this theme, see Henry James.) For example, a song on American ragtime begins with the declaration, "Grand opera stuff -- it's all a bluff." But, then, so is good old American treacle. LITTLE JOHNNY JONES -- At the Opera House through February 7.