So… I guess the first question is if the books should be written in German or if a good translation is enough. I’ll assume the latter and just won’t look whether a book was written in German or just translated.

Books aimed at children are a whole industry, more or less. Kids that learn how to read are their own target group, so you’ll find any genre you want. However, as the children are supposed to easily identify with their protagonists, they are often about school children doing stuff. More often than not about kids that read a lot and are quiet outsiders. If you can’t bear with that trope it’ll be a bit harder, but still manageable.

For criminal stories, there are some franchises that have been going on since more or less forever… “???” (“die drei Fragezeichen” and their “kids” spinoff that is targeted at younger audiences) or “tkkg” come to mind. A very German classic is “Emil und die Detektive”, which plays in Berlin in the 1920s and holds up very well.

If you’re more into fantasy, maybe Michael Ende’s more famous works are more to your liking. “Die unendliche Geschichte”, “Momo” or “Jim Knopf und Lukas der Lokomotivführer” are classics as well and just excellent. Cornelia Funke also wrote neat books, “Tintenblut” or “Drachenreiter” work really well. Recently, “die Schule der magischen Tiere” was pretty successful and my daughter loved the series.

Astrid Lindgren has good translations. All her stories are terrific. Pick anything. Some “Michael aus Lönneberga” book maybe.