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.
I don't think 'child soldiers are regular combatants' is actually the open and shut case you present it to be. I think the following is a more accurate portrayal of things (2016) - https://rsc-src.ca/en/voices/targeting-child-soldiers:
Despite the numerous volume on child soldiers in legal literature over the last few decades, very little has been said on targeting child soldiers. It seems to be something international lawyers would rather not talk about. The fact that legal literature doesn’t say much about targeting child soldiers doesn’t mean that no such practice exists, or that soldiers haven’t discussed the matter.