Do you know how deployable (or not) these ‘national guards’ are?
In the US case it is definitely possible (I don’t think it’s first-choice; but there aren’t any particularly substantial barriers and it has and does happen for our past few imperial adventures) for a ‘national guard’ unit to be activated and sent off along the same lines as a regular unit. It’s not an ‘in case of invasion of American soil’ thing.
Allowing under 18s into a national guard with those deployment options would be pretty straightforward use of child soldiers.
If, though, it’s more of a ‘for use only under the circumstances where we’d be drafting everyone who can hold a gun anyway’ type national guard it probably gets judged more along the lines of ROTC/JROTC type programs, which certainly encourage under-18s to head toward the military; but don’t actually allow you to use child soldiers under standard conditions.
Given Europe’s history of fairly apocalyptic wars, often fought on the territory of the combatants rather than some hapless proxy, I’d be fairly surprised if they’ve actually taken total-war type practices entirely off the table, even if they agree not to use them under ordinary circumstances.
