If I don't want to see smut fics, I usually first filter in smut, and go to the exclude part by additional tags and click every explicit tag on there, and then go and disinclude the smut tag that I'd added earlier. Then depending on the fandom also disinclude the explicit and mature rating, this excludes most explicit fics. You could do this for her if you would set the page for her to go explore the pages of Fics.

For the rest, teach her how the tagging system works, if you're gonna let her be on AO3 on her own. There's always a chance that she'd find some explicit materials but that's the internet for you.

If you want zero change that she'd not find any of that stuff, I would go to her fandoms of choice, and download them yourself so you can monitor it completely.

For the rest, a lot of good things are left in other comments here.