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.
You appear to be a strong analytical type. You have to remember that these people aren't argued into what they believe ... you probably were so talking to them is likely to be very frustrating and you will feel they are being dishonest (which they are, but they don't view truth as you do). I suspect you would choose to know the truth even if you discovered something horrible, such as your life had no meaning. Most religious people are scared to death of that.
I think the root of the matter is you have to discuss faith (pretending to know things you don't know) and bring attention to it as an unreliable epistemology. Well maybe it could be true ... well that's true of everything, SDBH mods could all be rothschild family members planning to take over the world too ... etc.
I've mostly not tried to help these types over the last few years, I think they know they are pretending, if it were any more than an emotional need they'd have spent a good 45min researching the subject (and I can guarantee you your friend here didn't go to the library and say "Yes I'm really superstitious, I was hoping to find a book by an atheist who used to share my beliefs but no longer does, probably with lots of credentials and experience". However, that is the absolute first thing anyone on an honest quest would do, and that's EXACTLY what you would do though you probably already did it (or maybe figured it out yourself, I think you said elsewhere you'd been in third world countries you can see how humans think).
Oh if they are religious and pentecostal you could show them Marjoe.