One way to make this argument is to say that parents, and not the government, should be in charge of deciding what commercials and how many commercials their children should be watching. In other words, you can make this argument based on the idea of familial rights and responsibilities. If parents do not want their children to be exposed to ads, they should regulate the amount of TV that their kids watch.
Another way to make the argument is to say that there will be no programming or magazines for children if ads are not allowed. Ads pay for the television programs and the magazines that kids watch and read. If kids were protected from the ads by law, there would be much less in the way of funding for children's media.