Walt Disney's (DIS) plan to prevent food companies from marketing unhealthful products to children over its media networks and at its theme parks is a game-changer.

The parent of the Disney Channel and ABC Family will require that food advertised on its networks meet a strict set of nutritional guidelines. Many popular products, such as Kraft (KFT) Luchables, along with fast food, sugary cereals and candy that are current advertisers, don't meet the guidelines, according to The New York Times. In addition, Disney will reduce the amount of sodium in children's meals sold at its theme parks by 25%.

The plan will be announced Tuesday at a press conference in Washington, D.C., with First Lady Michelle Obama. It will pressure Disney's rivals, such as Viacom (VIA.B), to adopt similar standards, and it will force food companies to rethink how they create and sell their products.

For instance, Kellogg's (K) may find it harder to introduce a variant of Pop-Tarts, whose current flavor line-up includes Frosted Chocolate Chip Cookie Dough, as it tries to compensate for lackluster sales of ready-to-eat cereals if it has fewer places to promote the product. Cap'n Crunch and other brand icons associated with marketing nutritionally dubious offerings to children for decades will probably need to be retired.

DreamWorks (DWA) is promoting "Madagascar 3" in McDonald's (MCD) Happy Meals, which often include toys tied to movies aimed at children. The Center for Science in the Public Interest (CSPI) and other members of the so-called food police have argued that these promotions encourage children to make unhealthful food choices, a relationship that I believe is not as clear-cut as some people argue. Since Disney ended its promotional agreement with McDonald's in 2006, DreamWorks may have a harder time justifying its relationship with the burger chain, as may Viacom, the corporate boss of SpongeBob SquarePants, which ran a Happy Meal promotion earlier this year.

Though CSPI lost the Happy Meal legal battle and will probably lose the battle to levy anti-obesity taxes on soda, the nonprofit and its allies are winning the bigger war. Food companies are making their products healthier. Now media companies will find it harder to accept advertising for products that are of questionable nutritional value.

Unfortunately, no amount of government pressure or industry public relations can make parents always act in their children's best interests. The obesity epidemic is a complicated problem that defies easy answers.

Jonathan Berr is long McDonald's. Follow him on Twitter@jdberr.