Come on, University of Kent, why the grovelling retreat? Your creative writing website got it right first time. You know perfectly well that when you made a distinction between "great literature" and "mass-market thrillers or children's fiction", you were standing up for something. That Keats is different from Dylan, or, in this instance, that Philip Roth does say something rather more challenging than JK Rowling, that Jonathan Franzen does create storylines more ambiguous and questioning than Stephanie Meyer's. What's so wrong with that? I'll go forward carrying the banner even if you won't.
Like Kent, we at City University take on creative writing MA students specifically to write literary novels – so we are quite ready to define what's required to write for adults as opposed to children. It isn't about the quality of the prose: the best children's books are better structured and written than many adult works. Nor is it about imaginary worlds – among the Lit Gang, for instance, Kazuo Ishiguro, Cormac McCarthy and Michael Chabon have all created plenty of those. It's simpler than that: a novel written for children omits certain adult-world elements which you would expect to find in a novel aimed squarely at grown-up readers.
The problem here is that Myerson fails to see that self-consciously “adult” novels, while they are indeed open to experiences, and to techniques, that children’s lit doesn't reckon with, also have blind spots, vast areas of human experience of which they are apparently ignorant. The estimable Adam Roberts covered this just a couple of months ago in an absolutely brilliant blog post that I wrote about here. The elaboration of "ambiguous and questioning ... story lines" may be a literary virtue — though perhaps not one that Jonathan Franzen possesses — but it is certainly not the only literary virtue or an indispensable one. The novel that is self-consciously for adults isn’t more comprehensive than the novel that is self-consciously for young people; it just covers different things. And, as Roberts makes clear, it habitually omits some of the most important experiences of life.