From a grammar point of view you’ll find that you want a lot of the things you’re going to learn in Genki II. You can look this kind of thing up as you go along, but if you want a smoother start then already knowing them will make life a bit easier.
Reviews for the book on Natively 魔女の宅急便 1 | L25 highlight that it’s fairly kanji light (being a children’s book) so pure kanji knowledge is probably less important than grammar and vocab.
There was a WaniKani book club for this a few years back: the threads will likely have some useful resources as well and discussion and questions/answers that might be helpful: