I haven't messed with the setting, but I have had a few items loaded via Calibre end up showing under Documents, and others under Books. I haven't figured out what about my library or the way I'm side-loading is yielding which results.
The OP should note that if you do NOTHING, the worst that will happen is your daughter will need to look in two places to find all her content.
It's not that you need to modify the tag to put the books on the Fire, you just need to modify it to control if it appears under Books or under Documents.
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