He thinks the movie does a disservice to kids by presenting what is, in his opinion, a distorted and sugar-coated representation of mental life and motivations.
Essentially, he thinks that a good movie would pass to viewers what Malick's Tree of Life does about the inner life of children (feelings of wonder, the need to act out for its own sake, the deception ones does for others and even oneself, etc) in a way that would appeal to kids.
(By the way, this is just what I understood from the review; I haven't seen Inside Out)
Isn't that exactly what he disagrees with, though? I think he's looking for a director that can tell a kids' story, and show the shape of the inner mind, without having to dumb things down.
It's a tall order - he says it would require a director of genius - but I think his point is that if a director needs to dumb things down to make a film for kids, (s)he's better off not doing it at all.
In Australia we used to have Margaret and David before they recently retired. They were really good film critics with tastes that covered a large spectrum of movies.
As people whose work is very subjective, critics don't lend themselves to be read in an ad-hoc fashion, in my opinion. One should follow their reviews, compare them with the works one has also watched/read, and only then decide which critics are more appropriate for you.
This is the author's justification for why he's critiquing the film. I disagree, personally, because he himself is not a child. I doubt any child would enjoy "The Tree of Life".