I'm sure many adult learners have done it: started learning a new language and bought children's books in order to aid with their learning. I've done it. One of the first Welsh books I bought, besides a dictionary, was "Ydw i'n fach?" by Philipp Winterberg.
It's a beautifully illustrated book designed to teach children new adjectives of size, and to teach them that although we may be different, we are all perfect. It's a lovely book.
But it's not suitable for adults. Once you've learned a handful of adjectives for big things, and a handful of adjectives for small things... that's it. It's a trap I'm sure many learners have fallen into: aiming too low. We think our limited vocabulary as learners mean we should buy books aimed at children, but ultimately there's no benefit in this. The vocabularly we need as adults is very different to the vocabulary we need as children, and we need to learn to structure sentences the way adults do. You can't get this from simple, children's picture books.
The next mistake is to realise the first mistake and over-correct by buying books aimed at older children. The problem here is that native-speaking older children, for whom the *subject matter* may be closer to that of an adult, have a vocabulary and comprehension of the target language far better than that of an adult learner.
Where Welsh excels is with its glut of excellent books aimed directly at second-language adult learners of all abilities. There's an excellent range of novels written by a pool of talented authors that address exactly the issues faced by adult learners of a language: that the range of vocabulary is appropriate, the pace of learning is appropriate and the subject matter is appropriate. And generally these books, unlike books aimed at native speakers, include a glossary of new terms at the bottom of each page as well as, often, a complete glossary at the end of the book. Publishers like Gwasg Gomer and Atebol produce books aimed specifically at different levels of learner, with each book marked to show at which level learner it is aimed: mynediad, sylfaen, canolradd, or uwch, so that a learner of any ability can find something to enjoy. Some books, like Ffenestri by Lois Arnold, contain a collection of short stories and poems broken down into sections with each section aimed at a different target level, so you can stick with the same book as your ability improves!