I've faced this kind of attitude before. As an author of children's books, it doesn't amuse me very much.
There is a grain of truth to it, in that you definitely need to write in an "easy" language, so that kids can understand. Also, you need to take into account that they are still shaping their reading habits, so you can't throw a postmodern, fragmentary experiment upon their heads, for example. You must more or less stay in the traditional forms and the common words and expressions. So, it's "easier" to write for children.
What people don't realize is that it is actually harder to write this way if you want to tell a good story. "Easy" writing tends to be bland, and you need to fight that blandness with its own weapons; you can't go to your arsenal and grab the brilliant adverb or the convoluted sentence. You must use the tools of blandness (common words, plain constructions, expected paths) to defeat it and transcend it.
What it tends to happen, I think, is that there are lots of bad writers, and these writers end up writing books for grownups with the same language and structure one would use to write for kids. That writer might well think that since he or she is using those tools to tell a complicated story, then it must be easier to write a less complicated story using the same tools. And in a way that's true: it's easier to be bland and basic than bland and pretentious.
By the way, thanks for the comic! I take it as a suggestion, and I plan to use that line the next time I'm involved in that kind of situation. Will report.