Santa was the only thing we ever taught DD8 that wasn't true, and drawing those lines between fantasy and reality has always been a help to her in enjoying sci-fi/fantasy/horror content well beyond what you'd expect for her years (last week, for example, she enjoyed The Omen). It doesn't scare her at all. Current events have the kind of impact you'd expect on an 8yo, because that's real. DW and I have to be careful what we discuss with her in earshot in that regard, and if DD hears something she doesn't like, she'll demand we stop talking about it.
My own DD had the opposite of your existential crisis, though... she had an epiphany on a topic she'd been bothered about for years. We're a nonreligious family in a religious area, so we had to address the subject with her at an early age, from our perspective. This was leading to some intolerance on her part, and some rather pointless arguments with her playmates, because she couldn't understand why some people chose to believe. When she decided "I want to believe" in Santa, I analogized that attitude with religious beliefs, and that was something she could understand and respect.
