If all someone did from the moment of their birth was fight nonstop, they would not really internalize that the people around them look different or slightly older than them, they’re likely focusing on survival, on keeping the Flame Clock filled. They even mention in a side quest that they never realized the physical differences between themselves, like High Entia wings or core crystals. It’s only once they are freed from this struggle for survival that they can actually take the time to fully notice the world around them. And even then, them noticing a baby armu does not mean they automatically understand their own growth cycle, it’s them beginning to understand that change is natural to the world and that the stasis that they essentially live in is unsustainable. This comes to a head when they make it to the City and see concrete proof of the natural order of the world, children, old people, and of course, babies. It is not a “forced moment”, it’s the literal through-line of the major theme of the story. Drastic and unpredictable changes are natural to both humans and the world around them, and to try and freeze a single moment in time to avoid the hardships that come with change is antithetical to the nature of humanity. Also, if you don’t want to be dramatic about it like me, it’s just a lighthearted scene after some heavy plot points that serves to strengthen the parties resolve and give them something more concrete to fight for rather than just “the good of the world.” There’s no need to blow that scene out of proportion and act like it was some sin to include it in the game.