It might be worth looking at the children’s novel club on this forum or its predecessor, the children’s book club on the natively forums.
In addition, sometimes the big publishers take some of their light novels etc and publish junior editions that have much more furigana.
If you’re browsing Japanese language books in a physical store, it’s worth mentioning that it seems to be a common trend to highlight these kid targeted books with a full colour border around the edge, though the colour choice varies by publisher.