It's not really part of the game, but that's what's so great about MGSV. If you played the original MGS remember how much fun it was to put C4 in the toilet when they go to take a crap? The entire game is like that! As such the complaints about the story being flimsy are justified, but c'mon, you're leading up to the events in Metal Gear for NES.. What happened to the child soldiers? Right now I don't care because I'm sliding around in a box in Outer Heaven and putting wild animals on balloons.
Big Boss: You [Solid Snake] saw those children, didn't you? Every one is a victim of a war somewhere in the world. And they'll make fine soldiers in the next war. Start a war, fan its flames, create victims... Then save them, train them... And feed them back onto the battlefield. It's a perfectly logical system.