With reading lots of children's literature, one thing really stands out: New books don't hold a candle to the old ones.
Even modern books that I like are not written nearly as well as the old ones are. There tend to be a lot of slang terms, bad grammar, a lack of punctuation, simplified sentence structures, and poor editing.
Reading an older book like "Ann of Green Gables" or "Pippi Longstocking" really shows the difference. The writing is richer, more complex, better thought out, and more standard in its grammar and style.
Few modern books can really compare to the old stuff. Even the Harry Potter books, which are among the best written of the modern books I've read, desperately could have used editing--"The Order of the Phoenix" and the later books in particular.
The sad thing is, I don't think this is just a matter of cutting out the editors in favor of cost-cutting. I think it is a general deterioration in our society's ability to write.
As much as I like the Percy Jackson books, I can't help but note that they are actually not written very well. And the author is a former junior high English teacher!
If the teachers don't write well, what hope for their students?
Writing really well, not merely telling a story and putting words on a page, may be a dying art.