"What constitutes a child soldier is clear and Khadr did not meet it according to the UN. He pled guilty to terrorism, and having blood on his hands, and showed remorse to his victim's widow and her children."

The UN disagrees with you. You can see the Convention on the Rights of the Child and children in armed conflict here:

http://www.ohchr.org/EN/ProfessionalInterest/Pages/OPACCRC.aspx

"Article 4.1   Armed groups that are distinct from the armed forces of a State should not, under any circumstances, recruit or use in hostilities persons under the age of 18 years."

"Article 6.3   3. States Parties shall take all feasible measures to ensure that persons within their jurisdiction recruited or used in hostilities contrary to the present Protocol are demobilized or otherwise released from service. States Parties shall, when necessary, accord to such persons all appropriate assistance for their physical and psychological recovery and their social reintegration."