I'm assuming you are not being ignorant of what a child soldier is on purpose?
Terrorists are not soldiers. This is well-established. It's why the ruling that Khadr's rights were violated was so utterly absurd. Terrorists are not covered by the Geneva Conventions. They don't meet any of the requirements, much less all of them. They have zero rights under international law. Calling him a child soldier is simply false. He was never a soldier of any kind. No more than a school shooter is. Picking up a gun under 18 or 16 or whatever doesn't make you a soldier. Serving in a recognized military does.