The government makes decisions about what kids can do all the time. Kids have to go to school, for example, or be home schooled. You can't just magically decide that schooling isn't for your kids. The movie rating system is already in place, would you argue that kids be allowed to see R rated? Certainly kids shouldn't see X rated stuff. Smoking is a lot more dangerous than picking up a cursing habit, makes sense to make movies R rated, i.e. "opt in". Meaning you can let your kids see R rated films, you just have to be there with them. Make smoking films R rated, your kids can still see them with your permission.