Children’s literature includes: (1) picture books for children who do not yet read (ages 2-6), (2) simple stories for beginning readers (ages 6-9), and (3) stories and non-fiction for children who are already reading on their own competently (approximately ages 8-12). The span of grammar, vocabulary, and and overall literary quality is great.
(1) The first category includes simple grammar and repetitive vocabulary. Yet children in this age group already know orally and correctly use basic grammar (e.g., simple verb tenses, common expressions) and vocabulary pertaining to their immediate life experiences. They can handle new words about things not yet experienced (far off kingdoms) because they are receiving explanations from their parents who are reading to them. By contrast, adult beginners don’t have this “base” from oral speech so all of the grammar and vocabulary is new. In general this level of children’s books is too hard for an adult learner just starting out. (Children’s books which just show individual words with a picture – e.g., colors, animals – although relevant, are boring for most adults.)
(2) The second category includes the simpler grammar and vocabulary that adult learners need but the subjects – about animals and fanciful characters – may or may not be that interesting for adults and still may not be than common (high priority). In Russian, the books at this level contain accent marks which IS really helpful.
(3) This category includes a broad spectrum of plot lines and sophistication. Some, like fairy/folktales, are classics and interesting but they use vocabulary and grammar that is not basic for modern adult learners. Depending on the plots, adults at a high intermediate level or higher may find them useful.
In general, children’s books are not teaching basic vocabulary and grammar since children get that information orally from family and friends. Rather, the plots are vehicles for talking about values, traditions, social norms, behavior, and emotions in ways that the child will find interesting and instructive. This is characteristic of traditional stories, legends, and myths cross-culturally, whether spoken or written.
Personally I have taken out children’s books at various levels in Russian from the library and found the picture books helpful when I reached an intermediate level but I still find Harry Potter too difficult to read comfortably.
Instead what I have found to be the most helpful are popular magazines in the foreign language. They have loads of pictures so one can even learn from the advertisements and they talk about contemporary subjects. Moreover, one can copy an article from their website online and paste it into LingQ, turning it into a lesson. Magazine articles were too difficult for me at the beginner level but are comprehensible for me at the intermediate level and generally are easier than newspaper articles.