A nation of tots on the edges of their chairs: Who'll replace Bozo?

''I always knew these would be big shoes to fill, but this is ridiculous!'' That's what Allen Hall, producer of the most successful children's show in Chicago television history, has learned in recent months. He's been searching for the man to fill the colossal clodhopppers -- both literally and figuratively -- of Bozo the clown, alias Bob Bell, the man widely acknowledged as the best of the hundreds of franchised Bozos around the world.

Mr. Bell, known as ''the king of the cloned clowns,'' retired here at WGN-TV in April after 23 years of performing jokes while wearing a crimson-dyed Tibetan yak-hair wig that sprays out from his temples like tufts of cotton candy. Producer Hall wants a new Bozo by July 1.

''Everybody in this country who ever smeared on whiteface and donned a funny nose has come in here wanting the job,'' says Hall -- who, with a 10-year waiting list for tickets, has the hottest show in town and is getting anxious about a replacement.

''The Bozo Show,'' which airs from 7 to 8:30 weekday mornings on Channel 9 here, draws as many Chicago viewers as NBC's ''Today Show,'' ''The CBS Morning News,'' and ABC's ''Good Morning America'' combined. It is also beamed or cabled to 4,000 other markets around the globe. In Chicago itself, parents write in for studio tickets even before their children are born.

Hall has been auditioning up to 12 Bozo aspirants a day at the studios here on West Bradley Place, as well as viewing the videotapes that others send in. ''They're coming from Ringling Bros. and Barnum & Bailey, the Shrine Circus, you name it,'' Hall says. They include self-employed mimists, jugglers, magicians, ventriloquists, and children's entertainers from all over. Hall won't disclose the salary, but cagily observes that ''it will be very, very comfortable, not including the up to five grand per day he could make in public appearances.''

He has a lot of talent to choose from. Apart from the 200 full-time, professional circus clowns in the United States, there are 35,000 semiprofessional and part-time clowns, according to Clowns of America, whose membership has surged in recent years.

''Clowning in America was a dying art until about 1968, but there's been a complete turnaround,'' says master clown Frosty Little, director of clowns at Ringling Bros. He says that when Ringling opened the clown college in Venice, Fla., that year, the ranks of its own clowns had dwindled to 16 -- aged 50 to 80 . Now it has 52, and the college feeds fairs, carnivals, and rodeos all over the country, which in turn generate more interest.

Bob Bell, who was a station announcer with no experience as a clown when he took the Bozo role in 1961, had no such college to attend -- and nothing like the resumes of the candidates seeking to replace him. One recent spate of would-be Bozos included a Marcel Marceau protege with 10 years' experience on three continents, a clown-college graduate who was also the gymnastics champion of Illinois and the mime champion of Hawaii, and a first-rate, 25-year-veteran character actor from Chicago.

But ''what most of these candidates don't realize,'' says Hall, ''is that we don't need someone who can pantomime, juggle, fall down, and do handstands -- all those things you usually associate with professional clowning. We need a good comedic actor who can improvise comic skits and talk warmly to kids on their own level.''

The success of the show, he feels, rests on the actor's ability to make the sketches come alive -- engaging children creatively, rather than passively introducing cartoons, telling prewritten jokes from cue cards, or selling products. Hall says those kinds of shows started dying when the overcommercialization of children's shows forced adoption of guidelines that prohibited pushing products as part of the show.

The original Bozo was developed by Capitol Records in 1940 as a voice in a series of storytelling albums for children. With the advent of TV in 1949, Capitol hired Larry Harmon, who created the costume, commissioned animated cartoons, and syndicated the shows to local stations, which hired their own Bozo hosts.

''In addition to the skits, we had an audience of 200, a 13-piece live band, we brought in circus acts, vaudeville acts, magicians, ventriloquists. And our guys were very witty -- without that, you'd flop,'' Hall says.

WGN's show survived, he says, because of the commitment to creative programs -- live skits that set the show apart from the dozen or so remaining Bozo-style shows nationwide. This is down from a one-time peak of around 80 stations -- including some in such faraway places as Ireland, Thailand, Greece, and Australia.

Like Bell, the other members of the WGN cast are also longtime veterans. They include Cooky, a clown who is the short and plump foil for Bozo; Wizzo, a magician-clown in Arabian Nights costume; and emcee Frazier Thomas, who plays straight man to all three.

In a typical sketch from a recent show, Bozo asks all the boys and girls in the live studio audience to help look for Cooky's four-leaf clover, while the three-piece band plays, ''I'm looking over a four-leaf clover.'' After much hubbub, emcee Thomas suggests thinking back to where Cooky lost it. Cooky replies: ''I lost it in the elephant tent.'' ''Then why were you looking for it here?'' he is asked. ''Because the light's better here,'' says Cooky.

Each year, that sort of humor brings some 40,000 kids to the studio here -- 200 at a time for the 90-minute daily show. The show is filmed twice each on Mondays, Tuesdays, and Wednesdays. Five are shown the following week and the sixth is saved to lengthen the season, which ends in April. Reruns are shown through September.

Not surprisingly, the show has had its critics over the years. '' 'Sesame Street' it ain't,'' said the Wall Street Journal last year. Several years before that, The New Yorker wrote of Bozo's ''carnival cheapness.'' But Marilyn Preston , TV critic at the Chicago Tribune, says that ''kids respond so strongly to it that it's hard to be critical. It's much better than all the abysmal super-action and fantasy fare that permeates Saturday morning TV.'' She applauds the longevity of a TV figure for kids that is not here today and gone tomorrow.

Peggy Charren, president of Action for Children's Television, the nationwide consumer group that tries to encourage more diversity in children's programming, applauds WGN's daily commitment to live programming for children. ''Most of the broadcast industry is not interested in programs where clowns or actors or real people talk to children on their own level,'' she says. Noting the absence of a single daily or weekly television show Monday through Friday for children on any other commercial network, she says the station's commitment of four regular performers dedicated to varied content among the cartoons ''is terrific.''

Hall says he dreams of what he could do with the $25 million budget that supports the writers and producers of the award-winning ''Sesame Street.'' Until he gets one, however, his search goes on for a Bozo to replace Bell.

''One advantage that Bob Bell had over whoever will be the new Bozo,'' he says, ''is the wealth of vaudeville and burlesque shows that were around when he was growing up. The new guy won't have that kind of exposure to draw on, because it doesn't exist anymore.''