According to the Geneva Convention, Khadr WAS NOT a child soldier whe he killed an American soldier:

"Article 4.3.c of Protocol II, additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of Non-International Armed Conflicts, adopted in 1977, states "children who have not attained the age of fifteen years shall neither be recruited in the armed forces or groups nor allowed to..."