I’m sorry…I struggle a LOT with hiragana-rich children’s books! I used to recommend Kiki’s as an early book, because it was my first book, and I was clueless!
As an adult who learned Japanese by studying with kanji on Duolingo and Wanikani, I get a LOT of information from kanji (easily seeing where verbs and subjects are, hints about the meaning). I’m finding that studying this way, it was actually more advanced to not “need” the kanji! (contrary to how people teach kids who can speak but not read; I could read but not speak)
I’ve seen several places that did analyses that allow more rational selections. I’m blanking right now, except for Christopher Fritz’ Study Log (he developed some awesome tools) and … I’m wondering whether it was KoohiCafe as another place that keeps book rankings? They did analyses like “If I know half of RTK, what percentage of the kanji in this book do I already know?”
I’m actually still working my way through Harry Potter 1. I got distracted leveling up Wanikani (I’m on level 32) and finishing DuoLingo (I just started Unit 6) and trying to do more output.