But beside the children, there's another reason why people are willing to tolerate the censorship of movie marketing. They're evading both the nature of marketing, and that it's a necessary part of having free speech.
What is marketing? In simplest terms, marketing is the process of getting a product to the market, which includes communicating to the market the nature of the product. It's this second aspect that most people understand as "marketing". (Technically, that part is advertising and promotion.)
There is no force involved in a movie commercial. There's inanity, clichés, and bad acting, but there's no force. The act of seeing a commercial doesn't force a child into a movie theater. A child needs permission to leave his home, a means of getting to the theater, a means of paying for the movie, and the permission of a theater employee to enter the theater. A piece of film cannot compel these things to happen. Since there's no force involved, government has no business making laws in this area.
It should be obvious that communicating the existence and nature of a product, such as a movie, newspaper, or book, is a necessary part of having free speech. If the government says "You can say anything you want, but we'll decide to whom you can say it," it's clear that the speech is not "free".
Right now, government is trying to tell us that they're not interfering with free speech, just who hears the speech. That's like saying you're allowed to place a phone call to whomever you want, but you're not allowed to have the call answered. Since when does a phone call consists of dialing numbers and talking to yourself?
If "freedom of speech" meant the freedom to talk to yourself, no one would have free speech except winos and the mentally retarded.
Freedom of speech is the freedom to communicate with others. Finally, let's just note that there's a reason the Founding Fathers did not specify that free speech is only permissible "amongst other adults". Parents have control over (and the right to control) their children's access to speech.