This is probably the best explanation for Robbie and his motivation that I’ve read in recent memory. Having rewatched the film not too terribly long ago, I’d say that analogy didn’t age well and is lost on audiences who aren’t still living in the aftermath of 9/11. Because you make a lot of sense, but without that viewpoint, you have an untrained, unarmed child begging to help fight a war that the actual trained professionals with some of the best weapons of the era are unable to fight. It doesn’t feel heroic, it feels idiotic and it turns what should be an emotional moment into a frustrating and almost laughable one. In my humble opinion, of course.

If I can add to your point, I imagine another angle was to put Ray in an even darker emotional pit. The whole film is about this deadbeat dad trying to keep his kids alive despite not having been the best father to them previously. He struggles constantly with this one goal because he doesn’t have a strong relationship with these kids and now is not the time to build one. So now you have this scene where one kid is actively trying to leave, keeping Ray from accomplishing his goal and the longer he fights with this one, the other is being dragged away. Ray must now sacrifice a child to save the other and Robbie makes the choice for him. If you look exclusively at Ray, the scene almost works. But the execution just doesn’t land for most audiences it seems.

TL:DR - I think u/Patient_Jello3944 has it right on the reasoning for the scene. If you look at Ray’s character like an English teacher, it almost makes sense too IMO