@Green Witch,
There are two different issues here.
The first is what I can do as an individual parent to protect my kids from things that are unhealthy. As you point out; it is very difficult for individual parents to protect their kids from things that are ubiquitous in society. This is especially true when these unhealthy products are specifically marketed to kids for profit.
The second issue is what can we do collectively, as a society, to protect our kids from things that are unhealthy. As a society we have a far greater ability to counteract the manipulation of our kids and to work together to address common problems.
Again the cigarette example is perfectly applicable.
Not that long ago to keep our daughters away from cigarettes we would have had to "lock them away" from TV, most magazines, teen novels, movies and boys.
We as a society acted to change these unhealthy messages. In addition to an effort to change opinion, there was also a success drive to pass legislation banning the marketing of a harmful product to kids.
We have healthier kids today as a result.