Guan...you are right..In 1977, the Additional Protocols to the 1949 Geneva Conventions prohibited the military recruitment and use of children under the age of 15, which is now recognised as a war crime under the Rome Statute of the International Criminal Court (2002). It applies to both government-controlled armed forces and non-state armed groups.

The prohibition on the use of children under 15 was reaffirmed in the Convention on the Rights of the Child (1989), which also defined a child for the first time as any person under the age of 18. https://www.child-soldiers.org/international-laws-and-child-rights

One would think that ISIS is the primary violator of Khadr's rights by their taking him as a volunteer. If Canada were to pay anything it should be a recoverable amount funding a claim against ISIS and its financiers on behalf of Khadr and Canadian courts should move quickly to recognize the widow's claim prior to any payment.