As for why Beast Nero is a child: I wrote a post essay on it a few hours ago, but the gist of it is as follows: if Beasts are supposed to be the sins of Humanity, then Beast Nero represents the absolute depths of sexual depravation. Yes, I'm talking about pederasty. I believe the design choice was COMPLETELY intentional.
In most contexts, this would make some sort of sense.
However. Let's look at this from the perspective of Type-Moon, and Otaku culture in Japan.
Now, saying 'Japanese people are pedophiles' is an incredibly harmful and offensive generalized sterotype. It's also absolutely not true, and when it comes to the general public, like almost everywhere else in the world, pedophilia is incredibly frowned upon. Children don't stop being children once you reach those islands, no matter how much 4chan and the worst parts of Reddit want you to believe it.
Hence why I'm specifying Otaku culture (which is still a pretty broad brushstroke, but bear with me here).
When it comes to Type-Moon and Nasuverse works, I'd hardly think that they'd consider pederasty to be the absolute depths of sexual depravity, and if they do, it's embraced pretty openly.
Partially because Prisma Illya exists. Partially because one of the voice actors that works on. FGO and is part of their main PR radio show is a self-admitted shotacon. Look at the Tsukihime remake and what happened to Noel. Apocrypha's Jack the Ripper. Sitonai's FA artwork. Abigail Williams. Wu Zetian. Thinking back, I could have just stopped at Prillya. Lolicons and shotacons are a core part of their audience, and they've readily accepted it.
I do agree the design was intentional.
But I can also pretty confidently say that it wasn't some sort of commentary on depravity, and rather that it was a gift to the depraved.
