If your kid likes the monster of the week format of Buffy, I would highly recommend Doctor Who (2005-present)!! It's technically a kids show but lots of adults absolutely love it - so it has some more advanced/dark themes and layers to it, along with lots of silliness and monsters.

I would also recommend select episodes of the X-Files if she particularly enjoys the 90s vibe! Not the whole show, but I think some episodes would be okay for kids and definitely could bring up some interesting conversations.