I 95% agree for the "Most self-published books for children are pretty terrible."
I teach English. I've taught both high school, elementary, and ESL for both levels.
If you're going to write a children's book, keep in mind that there are a range of vocabulary that certain ages will know, and vocab that children cannot read. You have to have some knowledge of language acquisition for children.
For instance, my first grade student can read basic 'a' vowel sounds like "bat, can, day," and so forth. If you write a book using other vowels that she hasn't learned yet, she can't read it. You really need to consider the reading abilities of younger children, rather than trying to feed a moral lesson and merely lowering the level of your vocabulary in your book. If anyone aims to write a children's book at the primary or preschool level, you'd really have to look at the language arts curriculum and write according to what students should have learned by their grade level, to ensure that kids can ACTUALLY READ your books.
I have no comments for young adult novels since I've met students who can read proficiently at their level, to students who've only ever read the text messages of their friends and never read any proper text. However, we've done a literature circle (grade 9, age 14-15) on dystopian teen novels and students were able to notice similarities across different books (Percy Jackson, Hunger Games, Maze Runner, and Speak) and stuff like segregation into groups, adults generally being portrayed as the antagonist/useless were common.
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Other than that, most of the novels I read and keep up with these days are webnovels, so I'm not really too invested in paper publishing and writers should place some hope in web-only publishing.
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EDIT: I forgot I took a media studies class and my instructor has mentioned the proliferation of internet self-publishing. (He even made the class laugh by showing us a collection of "bogglingly-bad ebook titles/covers) We did discuss something that publishers essentially acted as some sort of quality control or a certain line of standard. I know that the public will make even indie stuff popular if it gains the traction, but publishers have experience, understand the market, and adopt your pitches because they know what sells, and will do their best to help you sell.