We hear confident claims that Omar Khadr should have been treated as a child soldier, as if international law imposed some such obligation on the US. It doesn't. The UN convention on the rights of the child binds only its ratifiers. The US (not to mention the Taliban) never ratified the convention. Since child soldiers appear only as an 'optional protocol' to the convention, the US can hardly have incurred an obligation to respect its provisions or heed signatories' complaints.
But that's not the most troubling aspect of the child soldiers' defense. The defense, even if valid, essentially abandons the field to United States' mouthpieces. It suggests that but for his age, Omar Khadr would have been guilty as charged. If you suppose his age is the only legal reason for letting him go free, the implication is that the US would otherwise be within its rights to convict and punish him for war crimes. This is arrant nonsense. Omar Khadr, child or adult, may not have been justified in fighting the Americans, and the Americans may have been justified in fighting him; indeed in their invasion of Afghanistan. But the American invasion of Afghanistan was certainly illegal, so to accuse Omar Khadr of war crimes is mere impertinence.
Invading another country is, under international law, legal only in urgent, imminent self-defense - that is, if it is undertaken to fend off an attack known to be conducted in hours or days, not months or years. The US never even claimed this. Even if they had, the idea that bombing and attacking the Taliban, who had offered to turn Bin Laden over given evidence of his guilt, isn't even a remotely plausible case of 'staving off'. How would bombing and attacking the Taliban have disrupted Al Qaeda plans, if these plans were so far advanced that an attack was truly imminent? You stop an attack by attacking or capturing the attackers, not by waging war against some people who were sort of associated with them. The invasion might have been a reasonable long-term strategy to combat a broad terrorist threat, but that doesn't even come close to urgent self-defense against an imminent attack by an underground, internationally based movement - one which wasn't known to be on the brink of launching such an attack. Again, to be clear: the invasion might have been 'justified' in some broad sense of the term. It certainly wasn't legal.
If the invasion wasn't legal, resistance to the invasion was at least not illegal: nothing in international law forbids countering an illegal attack. So when Omar Khadr tossed a grenade, not at civilians, but at heavily armed illegal invaders who had attacked his position, the idea that he could have been committing any kind of 'war crime' is ludicrous. Stop letting him off as a child soldier. Start noticing that as a member of a force countering illegal state violence, he was entirely within his rights under international law.