One is the rule based on self-defense. That rule is always in play and permits killing a hostile threat - an armed, hostile threat from a child is the same legally as an armed hostile threat from an adult.

In both of the movie scenes, the child soldiers were in attack mode against the Union cavalry in one case and the Red Guards in the other (a less aggressive attack in the latter case, but an advance none the less). So, the always in effect self-defense rule applied in both cases.

Both movies are set in civil wars. Since both groups of child soldiers involved uniformed military forces, the cadets could be regarded as designated hostile forces as to which kill or capture would apply regardless of an armed, hostile threat or not.

I seriously cited the two movies as examples.

Regards

Mike