St. Louis County Ordinance 20,193 made it illegal to sell violent video games to kids, or to let kids play such games. In a short and sweet opinion, a panel of the Eighth Circuit Court of Appeals struck down the ban as unconstitutional.
The first major part of the opinion is a ringing declaration that games are speech. Borrowing strongly from the Seventh Circuit's decision two years ago in American Amusement Machine Ass'n v. Kendrick, the court reiterated that interactivity does not take away from the expressivity of games. Indeed, it points out that Choose Your Own Adventure books can be just as interactive as some modern video games (and that's a sad commentary on modern video games, let me tell you).
The court then proceeds to the usual "strict scrutiny" approach for analyzing content-based governmental restrictions on protected speech. And while it's true that there's a strong governmental interest in the psychological well-being of minors, the court points out that the connection between video-game violence and psychological harm is tenuous at best. Similarly, helping parents raise their children is a good thing, it doesn't trump free speech on the facts of the case.
(Digression: gee, so the court is saying that the harm that a law subject to strict scrutiny would fix must be in some sense traceable to some compelling governmental interest, and must be redressable by the law. Someday, off in a dream world of legal rationality, these tests will be harmonized with the very similar ones that come up under the law that determines who has standing to sue.)
The portion of the ordinance that barred the sale of games with a strong sexual component to minors remains in force, since the video game makers didn't challenge that half of it. Sex sells, but violence sells more.