Spencer’s argument is not flawed; his line of reasoning cannot be reduced by fallacy.

However, children are undoubtedly the ward of their parents. The line cannot be arbitrarily drawn at any of the points offered by Spencer, for there are undoubtedly some children who reach manhood yet would not qualify.

Thus through intuitive judgments (heuristics) must be called upon. While certain rights must be inherent to man, the rights of citizens cannot be given to children. Their special status precludes the manifestation of all the rights guaranteed a citizen; and might I add, the children accept this situation willingly.

“The candid thinker will find himself obliged to concede this, when he considers the many perplexities which follow in the train of any other theory. For, if it be asserted that the law of equal freedom applies only to adults; that is, if it be asserted that men have rights, but that children have none, we are immediately met by the question—When does the child become a man? at what period does the human being pass out of the condition of having no rights, into the condition of having rights? None will have the folly to quote the arbitrary dictum of the statute-book as an answer. The appeal is to an authority above that of legislative enactments—demands on what these are to be founded—on what attribute of manhood recognition by the law of equal freedom depends. Shall the youth be entitled to the rights of humanity when the pitch of his voice sinks an octave? or when he begins to shave? or when he ceases growing? or when he can lift a hundred weight? Are we to adopt the test of age, of stature, of weight, of strength, of virility, or of intelligence? Much may no doubt be said in favour of each of these; but who can select the true one? And who can answer the objection, that whichever qualification is chosen, will class many as men who are not at present considered such; whilst it will reject from the list, others who are now by universal consent included in it?

Nor is this all. For even supposing that, by some undiscovered species of logic, it has been determined on what particular day of his life the human being may equitably claim his freedom, it still remains to define the position he holds previously to this period. Has the minor absolutely no rights at all? If so, there is nothing wrong in infanticide. If so, robbery is justifiable, provided the party robbed be under age. If so, a child may equitably be enslaved. For, as already shown (pp. 112, 134), murder, theft, and the holding of others in bondage are wrong, simply because they are violations of human rights; and if children have no rights, they cannot become the subjects of these crimes. But if, on the other hand, it be held, as it is held, that children have some rights; if it be held that the youth has an equal claim to life with the adult; if it be held that he has something like the same title to liberty; and if it be held (though not by law, yet by public opinion) that he is similarly capable of owning property, then it becomes needful to show why these primary rights must be conceded, but no others. They who assert that children are wholly without rights, and that, like the inferior animals, they exist only by permission of grown men, take up a precise, unmistakable position. But they who suppose children to occupy a place morally above that of brutes, and yet maintain that whilst children have certain rights, their rights are not equal with those of men, are called upon to draw the line, to explain, to define. They must say what rights are common to children and adults, and why. They must say where the rights of adults exceed those of children, and why. And their answers to these queries must be drawn, not from considerations of expediency, but from the original constitution of things.

Should it be argued, that the relationship in which a parent stands to his child, as supplying it with the necessaries of life, is a different one from that subsisting between man and man, and that consequently the law of equal freedom does not apply, the answer is, that though by so maintaining it a parent establishes a certain claim upon his child—a claim which he may fairly expect to have discharged by a like kindness towards himself should he ever need it, yet he establishes no title to dominion. For if the conferring an obligation establishes a title to dominion in this case, then must it do so in others; whence it will follow that if one man becomes a benefactor to another, he thereby obtains the right to play the master over that other; a conclusion which we do not admit. Moreover, if in virtue of his position a parent may trench upon the liberties of his child, there necessarily arises the question—To what extent may he do this? may he destroy them entirely, as by committing murder? If not, it is required to ascertain the limit up to which he may go, but which he must not exceed; a problem equally insoluble with the similar one just noticed.

Unless, therefore, the reader can show that the train of reasoning by which the law of equal freedom is deduced from the Divine will, does not recognise children, which he cannot; unless he can show exactly at what time the child becomes a man, which he cannot; unless he can show why a certain share of liberty naturally attaches to both childhood and manhood, and another share to only one, which he cannot; he must admit that the rights of the youth and the adult are co-extensive.

There is indeed one plausible-looking way of meeting these arguments. It may be urged that in the child many of the faculties of the future man are undeveloped, and that as rights are primarily dependent on faculties, the rights of children cannot be co-extensive with those of adults, because their faculties are not so. A fatal objection this, did it touch the question; but it happens to be wholly beside it. The fullest endowment of rights that any being can possess, is perfect freedom to exercise all his faculties. And if each of two beings possesses perfect freedom to exercise all his faculties, each possesses complete rights; that is, the rights of the two are equal; no matter whether their faculties are equal or not. For, to say that the rights of the one are less than those of the other, because his faculties are fewer, is to say that he has no right to exercise the faculties he has not got!—a curious compound of truism and absurdity.” (from: http://oll.libertyfund.org/?option=com_staticxt&staticfile=show.php%3Ftitle=273&chapter=6304&layout=html&Itemid=27 )

Unless Spencer’s argument is somehow flawed, I see no way in which the initiation of force against “children,” even by their parents can be consistent with libertarianism. Therefore, libertarians have the right to intervene and stop parents from aggressing against their offspring, just as we have the right to intervene and stop aggression when the victim is not related to the aggressor.