I think the belief in the existence of Santa Claus is far more reasonable than belief in the existence of unsubstantiated beliefs.
That would depend on if you would consider a belief founded in fallacies to still be, in its own way, substantiated. I mean, would believing in wizards after reading the Harry Potter book series be a substantiated belief? If not, then yes, unsubstantiated beliefs can and do exist. I kid you not, I tried to cast one of the spells in the book; my toy did not, in fact, levitate.
It doesn't matter what it is, a book, a TV show, a trusted adult, etc., something supported or proved to the child that Santa Claus is real, so their belief is substantiated, or else they wouldn't have formed that belief in the first place.
Then your definition of substantiated differs from mine. Which is fine, I don't think the official definition of that word demands one way or the other. Just for the sake of clarity, a claim you would view as substantiated by erroneous information and fallacy would be viewed as unsubstantiated by most people, including myself. Also, a rose by any other name is still a rose; the overused Shakespeare quote is to the effect of: if you count errors and fallacy as still being enough to qualify a belief as substantiated, that's not going to make that belief on the same level as one based in reliable evidence evaluated properly, even though you are trying to shove it into the same category.
Trusted adults in themselves count as evidence in a similar way that trusted eyewitness evidence counts as evidence.
No eyewitness, no matter how trustworthy, is considered to serve as strong evidence by themselves, because eyewitness testimony is consistently one of the most unreliable sources of information.
Again, to say that someone can believe something without evidence or have an unsubstantiated belief is to say that their belief is uncaused because whatever caused their belief substantiated it.
What if they think the evidence exists, but they were lied to? That would cause them to believe something that was unsubstantiated, because from their perspective, they falsely think it is substantiated. No one is saying people believe stuff without a reason to believe it; the category of unsubstantiated belief usually covers false information and fallacy as sources of belief. If it doesn't to you, that sounds like a personal semantics gripe, and it won't change the reality that those claims, whether you call them unsubstantiated or not, are still invalid.
Naturally, we don't form a belief until we consider there to be sufficient evidence and the harder something is to believe, the more evidence is required before we'll consider it sufficient.
Yup. Notice how that isn't objective.
However, it is not fallacious for someone to believe something based on what they consider to be sufficient evidence when you disagree that it is sufficient to form your own belief.
That is fair, but, some people are stubborn to the point that their resistance to an idea is in and of itself a fallacy. As in, there comes a point in which the evidence for something is so significant, that it is viewed as a fallacy not to view the logical conclusion that stems from it as the most likely one to represent reality. For example, you'd consider it a fallacy for someone to think I was male if they saw me naked in person, and had access to my medical records, and heard me identify as female, even though there is technically a very low chance that their conclusion of me being male does represent reality.
There is nothing fallacious about taking someone's word to be sufficient evidence.
It's so fallacious to take someone's word as sufficient evidence that the only reason we do it so frequently is due to convenience. People are that unreliable thanks to lies and distortions of perceptions and memories. When my biological father denied being my father in court, do you think it would have been smart to rely on his word instead of the DNA test?