Yeah, I've been learning German for about a year and a half now through a combination of traditional classes and duolingo. I'm roughly high A2, low B1. I've tried multiple times to start reading children's books (e.g. Die Kleine Hexe, Emil und die Detektiv, Das doppelte Lottchen, Der Räuber Horzenplotz) which are all, supposedly, for readers under 10. They have a surprisingly large required vocabulary and grasp on the grammar.
I think part of the problem is that adult learners learn things in a sequence. They learn one part of grammar, then the next, then the next, so that there are grammar rules and structures that you are almost completely unaware of until pretty far into your learning. Whereas a native child is exposed to the entire language all at once. They don't fully understand everything, but they are aware of all of the different structures. Children's books know this and are written with it in mind, as opposed to things like readers, which restrict their grammar to a particular level. I have a much easier time reading an A2 reader than any actual childrens book.
I am a native speaker in German and I can tell you that while all the books you listed are children’s books, they are all quite old, and the words and phrases they use are weird, to say the least. Maybe grab a Lustiges Taschenbuch? They’re available almost everywhere, fun to read also for adults and since they are comics, the accompanying images provide context.
I think part of the problem is that adult learners learn things in a sequence. They learn one part of grammar, then the next, then the next, so that there are grammar rules and structures that you are almost completely unaware of until pretty far into your learning. Whereas a native child is exposed to the entire language all at once. They don't fully understand everything, but they are aware of all of the different structures. Children's books know this and are written with it in mind, as opposed to things like readers, which restrict their grammar to a particular level. I have a much easier time reading an A2 reader than any actual childrens book.