quran 33:59, here and later "Umm Muhammad" / "Sahih International" translation :
O Prophet, tell your wives and your daughters and the women of the
believers to bring down over themselves [part] of their outer
garments. That is more suitable that they will be known and not be
abused. And ever is Allah Forgiving and Merciful.
there is word "nisa" used. what does it mean? let's look at http://corpus.quran.com/qurandictionary.jsp?q=nsw . you can see that at least at some places it means "female human":
quran 4:176 :
They request from you a [legal] ruling. Say, " Allah gives you a
ruling concerning one having neither descendants nor ascendants [as
heirs]." If a man dies, leaving no child but [only] a sister, she will
have half of what he left. And he inherits from her if she [dies and]
has no child. But if there are two sisters [or more], they will have
two-thirds of what he left. If there are both brothers and sisters,
the male will have the share of two females. Allah makes clear to you
[His law], lest you go astray. And Allah is Knowing of all things.
quran 7:127 :
And the eminent among the people of Pharaoh said," Will you leave
Moses and his people to cause corruption in the land and abandon you
and your gods?" [Pharaoh] said, "We will kill their sons and keep
their women alive; and indeed, we are subjugators over them."
so, you cannot surely interpret that 33:59 commands hijab only to pubescent female human. so, at least it is dubious, and so, acording to a hadith, we should move away from dubious things. i have found such hadith : https://sunnah.com/urn/678240 :
Al-Hasan bin 'Ali said: "I remember that the Messenger of Allah
(s.a.w) said: 'Leave what makes you in doubt for what does not make
you in doubt. The truth brings tranquility while falsehood sows
take in account that it is possible to judge actions of parents and actions of children. according to hadith cited in answer of oshirowanen, children are not guilty if they do not wear hijab and cannot be executed. but question is about actions of parents, and i answer about actions of parents.
so, i think, it should told by parents to female children from birth, (when they do not understand speech they should clothe them), and they should prepare the proper cloth, but the hadith http://www.sunnah.com/abudawud/40/52 disallows executing/punishing, so i think parents should not force their children with anything like serious execution like hitting or not giving some essential food.