Graded readers; children’s books are a bad idea because the vocabulary is designed for the adults reading the books to their children. Often unusual descriptive terms you wouldn’t use in natural speech very often. Try Mandarin Companion’s “breakthrough” level for stories told with 150 characters, they’re a great start. Might help to go through the vocabulary lists first on Skritter or something. Caveat: I’m a beginner, this is just what people much better than me have said.