I just did a fairly deep dive on this because it would be a horrible thing to argue about without a full understanding. You're incorrect - a child soldier is a combatant under every relevant international law I could find, from the Geneva convention onwards. Further, there have been numerous horrific cases of soldiers having to shoot armed children, none of which has been labeled a war crime.
A particularly concise quote from the European journal of international law:
The starting points are articles 4(A) of the 1949 Third Geneva Convention and article 43 of the 1977 Additional Protocol I, which provides for the definitional elements of what a combatant is under international humanitarian law, albeit overtly in the context of an international armed conflict. If a child is enrolled in the armed forces of a party to an international armed conflict, there seems to be no apparent basis in current international humanitarian law to characterize that child as anything other than as a combatant.