It's early on a Sunday evening, and time for a commercial break on The Muppet Show. And here it comes, kids, a high-powered pitch for . . . auto parts?

Until last year, that commercial spot would have been filled by an advertisement for an Atari video game, but then the government of Quebec Province threw the book at Atari's Canadian distributor, Irwin Toy Co. Ltd. The charge against Irwin: violating Quebec's strong, rigid ban on all advertising aimed at children under the age of 13.

Irwin, which faces more than 200 criminal charges of violating the 1980 law, is challenging the constitutionality of the provincial government's sweeping prohibition on ads directed at children. But in the meantime, the law is being enforced, and this sprawling province of 6.3 million residents has become the laboratory for a unique experiment in marketing and child psychology.

What is happening here is what might have happened if the restrictions on children's advertising contemplated by the U.S. Federal Trade Commission in the late 1970s had gone into effect. The results are inconclusive--there are no firm figures, for example, on how many of Quebec's French-speaking youngsters are watching American programs from across the border--but the anomalies are manifest.

"The Wonderful World of Disney" is sponsored by "Impulse, an irresistible fragrance you use all over." A French-language children's game show is sponsored by a paint company and a golf club. Cartoons on commercial channels on Saturday morning are run without advertisements. Only program notices and public-service announcements appear where Americans hear about sugared cereals. Quebec's children don't hear about violent or expensive toys, but they don't hear about toothpaste or fruit juice either, because the ban applies to all ads, not just those for products considered harmful.

All the arguments that raged before the law was enacted are still going on. Advertisers say the ban is unnecessary, economically counter-productive, and self-defeating because it drives the province's children into the cultural embrace of U.S. television. Supporters and enforcers of the law say it protects children too young to distinguish between program content and commercials, and restricts unwarranted intrusions by advertisers into the family.

Pierre Valois, director of Quebec's office of consumer protection and chief enforcer of the law, said it is needed to restore parental control over access to children's minds. "If somebody comes to your door on a Saturday morning to sell something and says, 'we don't want to talk to you, we want to talk directly to your children,' nobody would accept that," he said. "This law says 'don't intervene in the family.' Parents know what children need; tell the parents what you are selling and let them make the choice."

"The whole thing is a dog's breakfast," said J. Richard Genin, sales manager of Tele-Metropole Inc., operator of the major French-language commercial channel here. "We have an in-house screening committee that looks over every ad that comes in, and evaluates its content and the prospective audience to see if it complies with the code. We argue like hell and we waste a lot of time." He estimated that the combined cost to his station of compliance measures and lost advertising revenue is $1 million a year.

The law went into effect in 1980, and the provincial government has been haggling over its interpretation ever since with advertisers, advertising agencies and the television stations. The Irwin Toy case is the only criminal prosecution so far, but Pierre Valois, director of Quebec's office of consumer protection, said in an interview that he is "probably going to have to go to court soon" against some advertisers who he said are still not meeting the guidelines issued by his office.

Restrictions on advertising to children have a long history here. In 1972, the Quebec legislature wrote into law many of the restraints already adopted voluntarily by the Canadian Association of Broadcasters.

That law included a ban on the use of well-known cartoon characters to sell products to children. In a test case, Quebec went to court against Kellogg Co., the cereal maker, over the use of Kellogg's Tony the Tiger character. The Canadian Supreme Court ruled that the province had the right to exile Tony from the airwaves--a ruling that informed observers here believe presages a victory for the province in its current struggle with Irwin.

The 1980 ban on all children's advertising was part of a comprehensive truth-in-advertising law that covered everything from bait-and-switch ads to the techniques of used-car salesmen.

It provides that "no person may make use of commercial advertising directed at persons under 13 years of age," with fines for violations running up to $50,000. This applies to newspaper, magazine and billboard ads as well as television, but the home screen is the only battleground that matters to the advertisers. According to a Quebec goverment report, Quebec residents watch TV an average of 24.1 hours per week and "only one Quebecer in three reads a daily paper."

An "application guide" issued by Valois' enforcement office said any ad would be presumed to be directed at children if it included "themes related to fantasy, magic, mystery, suspense or adventure"; if it depicted authority figures, role models, heroes, animals or "imaginary or fanciful creatures"; or if it relied on cartoons, children's music, or attention-getting technical devices such as "spectacular sound and color."

Such ads may never be shown, Valois' staff ruled, in the early-morning, late afternoon or weekend morning hours when children dominate the audience, or during "family" viewing time when the audience is more than 15 percent children. Products "of interest" to children may be advertised, but only if the "treatment" is designed to appeal to adults.

The effect of this, advertisers say, is that advertising agencies have had to adjust their marketing techniques to sell through the parents, rather than directly to children. There is a wide range of opinion, most of it subjective, on the effect this has had on the marketplace and on the viewing habits of the province's children.

"The toughest challenge is introducing new products," said J. Brian Clarke, president of Coleco (Canada) Ltd., a major producer of toys and electronic games. "You take a novelty item with no appeal to adults whatsoever, like the 'Slime' toy of a few years back. It had no redeeming social value. You can't market that through the parents."

He also said that video games, which have largely entered the marketplace since the Quebec ban went into effect, have not sold as well in Quebec as they have elsewhere because they cannot be advertised directly to their consumers. A corollary of that, Clarke said, is that retail prices for electronic games are generally higher here because there are no recognized brand-name items that retailers can discount to lure traffic into their stores.

"No sense advertising a big discount on 'Choplifter' if nobody knows what it is," he said.

Samuel M. (Mac) Irwin, head of Toronto-based Irwin Toy, said there is "no question that toy sales have dropped considerably in the province of Quebec, for all toy manufacturers. It's not clear how much is due to the ban and how much is due to the terrible economy"--Quebec's unemployment rate is 16.1 percent--"but the ban is certainly part of it."

Kellogg executives said the ban has had the effect of locking in the share of the cereal market that each producer had when the ban took effect, because it is so difficult to launch a new brand. However, Michael Kennerley, advertising director for General Foods of Canada, said the Kellogg view is only a "good hypothesis," not yet proved. Kennerly, whose company has had disputes with Valois over its ads for Jell-O, said that "market shares are not permanently frozen. What we have to do is learn to market children's products to the parent who is going to buy them. We can buy prime-time TV and have adult-oriented commercials which are attractive to the children, too. It's a fine line."

"I don't think there's any real impact on market share," said Marvin Goldberg, a marketing professor at McGill University who has studied this issue for several years. "There are too many other ways to reach kids, and marketers are too clever. As long as there's a corner store on the way home from school, children can be reached."

Advertisers in all of Canada voluntarily adopted a code of restrictions that prohibited some advertising techniques that are common in the United States, such as encouraging children to ask their parents to buy a product. By imposing its legal ban on top of that voluntary code, the advertisers say, the French nationalist, separatist government of the Parti Quebecois is only driving French-speaking children to watch programs broadcast from New York and Vermont.

Madeleine Saint-Jacques, director of the Montreal office of Young & Rubicam advertising agency, said sarcastically that the law "has had one good effect. More and more children are learning English to watch the American channels."

Genin, the sales manager of the French-language station, said that the price of children's programming is going up, and "if we don't have Daffy Duck because it's too expensive, the kids will just watch it from the States. In English. With commercials."

Valois, however, said this is not true. A study done for him by a local audience-rating service shows that only one program received from the United States has more than 7 percent of the Quebec children's audience, while 37 shows that originate in Canada have more than 10 percent.