Restrict audiobooks on the books app on my child's devices/report explicit books that are not correctly flagged

One of my kids discovered that with the books app, set to "clean," still shows results for sexual/erotica/etc. topics will bring up a number of books that are "explicit" (thus no previews are available to them on a "clean" setting), but also a number of audiobooks that are extremely explicit yet not set to explicit, which allows kids to listen to samples of erotica.

Note: I have already chosen to set the bookstore to "Clean" in settings

I want to have the books app available on their devices so they can read books I've allowed them to purchase, but I don't want them reading erotica -- or especially listening to people reading erotica with all the dramatics that this entails.

Is there a way to flag books/audiobooks as explicit? My web searching informed me that the author is supposed to tag their material in the metadata, but it looks like a lot don't, and there's no obvious "report" button.

The audiobooks look way worse than the books in this regard, with almost none of them tagged as explicit, while the books appear to mostly be appropriately tagged.