Not my area of expertise, but my understanding is that this is an extremely limited claim (i.e. unborn babies / neonatal supplementation, or children with genetic defects), and not something you can even remotely generalize to "children" as a whole. The way we teach this in biochem 101 is "arginine is conditionally essential", with the "conditional" being "periods of stress or sickness".

In any case, I seriously doubt the author is talking about this, given the imprecision of the language. Also, eating this wheat specifically for arginine supplementation is goofy gym-science. Arginine is trivial to supplement.

https://www.sciencedirect.com/science/article/abs/pii/S03783...

https://www.sciencedirect.com/topics/medicine-and-dentistry/...