I mean, if you think about it, what does them being child soldiers add to the narrative (not what they hypothetically add, but actually add to the story), that them being regular recruits doesn’t? It was an interesting idea, but they never actually explore it, so much so that you could likely remove it and lose very little to their story.

Again, if anything, it would add more nuance to the story, because it gives Finn and Jannah more agency. They chose to leave the First Order, but only after choosing to join it. Their shame of being stormtroopers is even more justified, in this instance, because they have to accept they made a mistake.