Children's books? Children also learn from scratch. I think an adult reading books targeted to progressively higher ages would do reasonably.
assuming we are talking about children's content in general (and not content for babies), you will find that it is not much more comprehensible than the adult stuff.
a language learning beginner is nowhere near the command or vocabulary of a child.
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.
Getting to a middle-school or YA level though, and increasing through reading levels there, is probably the best thing you can do for your language (and phrasing was built specifically to make this level accessible to you before your language level is high enough).
PS: Childrens books work on Phrasing too, so this isn't inherently biased in that respect, just my genuine opinion.
Nothing to do when I was a kid, so I would not rule it out.