This isn't a child or anything, they're a grown adult.
I know a person who really believes in a lot of supernatural bullshit, and refuses to get back to reality.
Some notable things they've done would include spending a few hundred dollars on special 'healing' rocks, blaming everyone's behavior (including their own) on astrology signs, and actually warning me, as though they were concerned for my safety, about not getting into occult things (I made a dumb joke about Satan worship) because of this story they've heard about a guy buying an ouija board and having to sell it on eBay because some spoopy shit was happening.
Whenever I tell them that science has produced no evidence of any of these things, or that the easiest explanation to all of those things would be fake stories, ancient superstition, and the placebo effect, they'll just respond with "Well just because nobody's proven it yet doesn't mean it doesn't exist".
Then I'll ask them if we should believe in unicorns or purple elephants or something stupid, and just assume they haven't been found yet. To which they'll respond with a smug "Maybe, who knows? Nobody can know everything for sure."
How do I convince them that they believe in a bunch of delusional crap? I'm really worried they might fall down the wrong rabbit hole one day and end up getting hurt.
Look up the Invisible Pink Unicorn. It targets this exact thing.
https://en.wikipedia.org/wiki/Invisible_Pink_Unicorn