Children can cause just as much harm to others as adults, and if a parent is not willing or not able to discipline, then how else can society go about protecting itself? Of course people should exercise restraint with disciplining others' children since it may undermine their ability to discipline them in the future. Now I should be clear, when I say the government, I mean a branch of civil executive power with the responsibility to protect the society that it governs. We can argue the merits of which particular level of government to give this power to but this wasn't the question asked. At some point, a government will have to play this role.
Why should people under 18 be punished for things that no one would dream of punishing someone over 18 for? They are not subhuman! Adultism is a massive threat to society and ruins lives early on. Anyone who disagrees should go and jump in a volcano. There is NO scientific evidence that under 18s should be 'punished'
Of course children and teens should stay within the boundaries of the law, but it is not the Government's job to assign punishments and home rules. Parents may be doing wrong in letting kids watch violent movies or video games, and enforcing other bad habits, but it is not the Government's job to set the standards for morals and parenting. Of course, past a certain level of terrible parenting, the Government can take a child through social services, but it is always a bad thing for society when you let the state control the upbringing of children.