The author seems to like the books, but somewhat downplays the children's world and nature. From my understanding of the author's article, It's a nature he believes adults shouldn't have and yet powerful people do. So he's bringing this up, comparing the children in Captain Underpants with these powerful people. And also he's reflecting on how media is created with a "childish mind".

Personally, I don't think there's anything to downplay or wrong about children or being childish as adults. That's not the problem. The problem's the insensitivity and shamelessness of powerful people.