Is there a case for thinking that much of the available literary talent in the last century has been drawn to write for children? Perhaps. Why? Because in that realm they are almost free of illiberal interference, and as a consequence can write about the world as a physical experience, rather than a pretext for a delicately poised moral commentary, as is demanded in writing for adults. Freedom is a great magnet.
Then again, the high average quality of writing for the young could simply be explained by the fact that since the area is relatively unconstrained, even mediocrities can bloom in ways that first raters working within the narrow constraints of prestige intellectual composition could not hope to emulate.