There's not much to establish, given that there is no hard data on the costs of masking children. One of his qualms is that we're 18 months in and no one has bothered to get this data.
I think he laid out his case about as well as he could, given the data we actually have. He points to the established benefits of face time, as signed off on by the CDC and AAP, to show wha we are losing by masking. He then shows how masking little children under 5, and possibly even under 12, isn't recommended outside of this country and likely has minimal benefit to begin with.
Until we have hard data on the actual costs of masking kids, I don't see how anyone can make a stronger argument, either for or against.
So basically you agree with me. My issue is specifically with that lead line of the article that I quoted. It's possible that he didn't write that part, but I would think they don't do that at a place like the Atlantic.
To me establishing means as direct evidence as possible. If inferring from other 'established' concepts was sufficient, we'd have a very strong case that masking helps, since it's pretty well established in adults. He does a decent job of showing fairly direct evidence of the lack of benefit in masking kids with the study out of Spain, but provides no real direct evidence of the cost of masking kids.
Ultimately it shouldn't matter, since we basically do see that masking kids (especially the youngest) provides little benefit, while there's reason to suspect (even if not proven one way or another) that there are costs to masking. There's no reason to overstate the case, which I feel that opening line does.
He links to standard guidance from pre-covid, though I agree he could have given a few more examples to strengthen his point. There is plenty of literature regarding the importance of eye contact and facial and body expressions in various areas of childhood development, and pre covid this was a given, or "established". To my knowledge, there were no studies done showing the harms of not having that, because it would be highly unethical to potentially harm a child by subjecting them to such a study. What was taken as a given is now being sacrificed for little or no gain.
See above.
Also, as an aside, the one thing he does link to regarding the importance of children seeing faces is barely relevant. Very puzzling why he chose that. The population was specifically growth-stunted toddlers (ie kids that in our system would get services, or be in a special program), not general population, not school aged; the outcome studied was earnings as adults; and perhaps most importantly, does not mention anything about seeing faces.