JS Mill可能免除儿童伤害原则的原因有两个。
One is that children should not exercise their liberty to their own detriment, even when they would be allowed to if they were adults because nobody else would be harmed.
But a different reason is that children do not possess liberty at all, so that it would be impossible for the harm principle to apply to them even if we wanted it to.
Could this be Mill's real reason? I think so.
Allow me to quote Mill's entire paragraph about children that was partially quoted by John Perry above:
也许，几乎没有必要说这种学说只适用于能力成熟的人类。我们说的不是儿童，也不是低于法律规定的成年或成年妇女的年龄的年轻人。对于那些仍处于需要他人照顾的状态的人，必须保护他们不受自己行为和外界伤害的伤害。出于同样的原因，我们可以不考虑那些落后的社会状态，在这些社会状态中，种族本身可能被认为处于幼年期。在自发进步的道路上，早期的困难是如此之大，以至于很难选择任何方法来克服它们;一个充满改进精神的统治者在使用任何权宜之计时都是有保证的，这些权宜之计可以达到一个目的，也许在其他方面是无法达到的。专制是对付野蛮人的一种合法的政府模式，前提是其目的是改善野蛮人，而且手段是通过实际实现这一目的而正当的。自由作为一项原则，在人类能够通过自由和平等的讨论而得到改进之前，对任何情况都不适用。在那之前，对他们来说，除了绝对服从阿克巴尔或查理曼大帝(如果他们有幸找到的话)之外，别无他法。但是，一旦人类获得了通过信念或说服来引导自己进步的能力(这一时期在我们需要关注的所有国家早已达到)，强迫，无论是直接的形式，还是对不服从的痛苦和惩罚，都不再被允许作为一种对他们自己有利的手段，而只有为了他人的安全才有理由。
很明显，密尔并不是在谈论孩子本身，而是在谈论那些"能力还未成熟"的人密尔写道，不仅是儿童，“那些落后的社会状态(种族本身可能被认为处于幼年期)”也不受伤害原则的约束，而且“出于同样的原因”。原因不在于他们拥有必须加以限制的自由，而在于他们根本不拥有自由。“自由作为一项原则，在人类能够通过自由和平等的讨论而得到改善之前，对任何情况都不适用。”
By the way, let me be clear that I very strongly disagree with Mill's racism, as I'm sure we all do.
In any case, Mill gives the same rationale for parenting children as for colonial imperialism: one is a benign despot/parent whose ultimate aim is to provide one's subjects/children with their own eventual liberty, of which they have none yet.
So the question for Mill is not how the liberty of children should be restricted. That's a loaded question which must be rejected because it assumes that children have liberty when in fact they do not. Rather, the question is how the liberty of children should be ACQUIRED. For the harm principle will not apply to them until they first have liberty to restrict, and they will not have liberty at all until they "have become capable of being improved by free and equal discussion."
Therefore, according to Mill, nobody possesses liberty by necessity, or by nature, but only as a matter of contingency, by nurture. Liberty is a skill which must be acquired, developed, nurtured. And like most skills, liberty could remain unacquired one's entire life if left undeveloped. Hence the intimate connection between education and liberty, and thus education and children.
Liberty, according to Mill's European intellectual tradition, is a power or capacity developed by means of education in the liberal arts, of which traditionally there were seven: grammar, logic, and rhetoric constituted a basic primary education, while arithmetic, geometry, and applied mathematics like harmonics and astronomy constituted a more advanced secondary education. Acquisition of these basic skills or "arts" was considered necessary in order to possess liberty. The idea was that in order to make genuinely good choices in life, and thus be genuinely free, one must be able to read, write, make and evaluate arguments, do basic arithmetic and deliberate with others -- or as Mill puts it, "become capable of being improved by free and equal discussion."
Now, I wish to make one suggestion: that in order for children -- or anybody else, for that matter -- to become capable of being improved by free and equal discussion, they must be taught how to engage in free and equal discussion in the first place. How many parents teach this to their children as a skill? On the contrary, many parents deny their children free and equal status in discussion! And if there is no free and equal discussion between parent and child, how can parents teach their children this valuable skill?
哲学在许多方面都非常适合教那些有充分认知发展的孩子们自由平等地讨论的技能。哲学通常不是关于那些可能会给孩子们带来麻烦的实际问题，而是关于那些可以训练孩子们的思维而不危及他们身体的假设或理论问题。哲学为孩子们提供了在实际问题上没有做出可能造成伤害的错误选择的风险的练习。
So parents, grab your Plato and start practising some dialectic, because you've got discussion with your children as free equals to attend to....
Cheers,
-paul

As a mother of 3 young children I have come up against these issues and had to develop my own philosophy. Most of the time I manage to abide by it but I admit at times I resort back to "because I said so" only because in that moment I am lacking patience. However, I do beleive that children must decide what is right and wrong on their own and no amount of me telling them will pursuade them unless they reach there own conclusions from personal experience. Personal experience sticks better in the memory. When using force or fear to control a child you are only creating another struggle within themselves and with the world around them. People tend not to think you can pursuade or reason with a small child but at 1 year of age I found patient explination was far more effective. If I can give them a good enough reason that makes sense in their perspective then they will go along but if I use force then I only get struggle and fighting. As the get older they need solid logic and reasoning so that they can talk themselves through situations that are challenging. However if it is force and fear that has been emebedded in their minds then as they get older they will only find loopholes to get around it. This forces children to lie and do things in secret so that they can get away with it. And if no one is around to rule them by fear then what will they rely on in the long run to stay out of trouble?
我真的相信，如果我们希望我们的孩子成为负责任、富有同情心和理解的成年人，那么我们就必须在他们的头脑中灌输一个可靠的推理过程，帮助他们用他们的头脑而不是他们的情感来克服困难。更好的是，我们应该教他们将两者结合起来，这样他们在决策过程中就有了一个双重检查系统。

我同意密尔的观点，我认为如果成年是成熟的衡量标准而不仅仅是年龄，那么更容易看到与自由的联系，那么不成熟的就不一定是孩子，而是任何出于自我满足或无知而继续伤害自己和他人的人。这个问题似乎与责任有关，例如，如果孩子渴望得到关注，他们会做任何事情来获得关注，这意味着实验，他们不理解自己的行为，因为他们没有足够广泛的经验来判断后果。因此，如果不给予适当的关注，就无法追究他们的责任;相反，责任被传递给了父母，所以孩子的主权实际上并不存在。
这一部分:“但是那些成年后没有养成这些习惯、技能和知识的人怎么办?”“我们知道发生了什么——我们看到他们在超市推着手推车，或在街上要求零钱，或在监狱，在法庭的电视节目，杰瑞施普林格节目，或在医院。
My question would be "At what point does a person stop being a product of their environment and at what point does the environment become a product of the person in it" I think that point where that happens is the dividing line between the child and the adult

If you're interested in these questions you might appreciate this famous PBS Frontline documentary:
http://www.pbs.org/wgbh/pages/frontline/shows/divided/etc/view.html
Cheers,
-paul

That Tamar Shapiro is pretty cute.