No need to get questionable about it Bezzo, if your going to pick at the rock of child soldiers in war and how its questionable in wargames then your going to bring down the entire mountain of military history and conflict on your head. None of its very pretty, and since the majority of males who fight in wars are 16 - 20, well no need to buckle any less. People could easily have a tantrum about all wars given the many 'injustices' committed, and many we play wargames about without truly understanding how brutal they were like WW2 wargames and we certainly don't question them ort find them repulsive enough for gaming.
No need to make an exception here if you ask me, we don't need to start a habit.
As for child soldiers, I don't know of any myself, since I have never looked, but I suppose using 25mm etc would be fine. Personally was never one for it with my modern games!