My bad for not having said it earlier : in reality, the artistic direction is perfectly suited, because this is what the developers wanted it to be. And really it's just my personal opinion.
It's very tricky. Somehow WW1 is still a subject somehow alive in our society, at least for what i know, in France; for various reasons, which could be political, cultural or familial. We may have known a member of our family who lived this war, or still feel its consequences on our current familial context. And thus i guess, somehow all share an intimate relationship with that period of history.
I do also like and approve the pacifistic take of it (although nowadays that would be difficult to advocate the contrary).
So with that said, what i don't like about the artistic direction of the game is not that I find it to be too "childish" (why not?); but really, too "pleasant". It's pleasant because it's aligned on a melancholic / heroic / meaningful vibe (which is very relevant in the game soundtrack) and do its best to bring empathic passions into the audience by the extensive use of pathos. It puts on scene these virtuous / candid / martyr but still fiercely courageous figures; the real heroes, resolute and just victims of the madness of men.
I think it should be the absolute contrary, if you would treat war artistically, the form should not be "pleasant" but absolutely "unpleasant". Unpleasant in unharmonious, metallic, white, nihilistic, rotten, absurd ways. Not trying to make the chords of lyricism resonate (rather make them resonate falsely) but expose war as you would throw blood and intestines on a white table. Not to create empathy but to create trauma, fear and disgust. Expose ugliness rather than making the beautiful passions resonate. Well at least that's my opinion.
To simplify really, i would adapt the first chapter on WW1 of Céline, "Journey to the end of the night".
And Maus is indeed really cool !