Late to the discussion, but to answer the OP's question:
"As long as he doesn't try to harm anyone including himself, what's the harm in him believing nonsense?"
The harm is that by giving the child incorrect information, he will eventually end up harming someone unintentionally in a matter that could have been avoided. That belief in nonsense does not stay confined to privately held beliefs that are never acted on. It gets into everything by it's very nature - anything offering values on the scope of a religion has to, else it loses all it's value.
Which means it gets into every part of their lives, and causes them to make decisions based on incorrect information. There are examples of this to be found everywhere. For a most straightforward example, take Christian Science's belief in healing through prayer, and not medication. This has resulted in children dying from treatable illnesses. In this case, the parents certainly did not intend to harm their child - quite the opposite. But because their understanding of the world was flawed, their intentions did not matter - their children came to harm.
That's the big problem. Religion, by it's very nature claims the entire world as it's scope of authority. Which means if you accept nonsense in it, that nonsense gets into everything. And then harm is going to be done, intentional or otherwise, simply because no one can make a wise decision based on nonsense.
Good answer, thanks for contributing.
But your model of a child sounds more like a notepad app on your smartphone or PC than an actual human being. As if it's going to just take in all the information given to it and not think on it's own.
The harm is that by giving the child incorrect information, he will eventually end up harming someone unintentionally in a matter that could have been avoided.
Really? He WILL end up harming someone? I find that hard to believe. There has been a lot of misinformed people throughout history that have not gone on to harm people. Most people, maybe even all people are misinformed about a lot of topics. Misinformation is prevalent, and yet people live fulfilling lives despite it. Especially if it's temporary. which it always is.

:hmm: Speaking of temporary...
For a most straightforward example, take Christian Science's belief in healing through prayer, and not medication. This has resulted in children dying from treatable illnesses.
Isn't this just an example of natural selection at work? What is wrong with that? It's natural. It's how nature works. For a most straightforward example of how nature doesn't work there is always Mars. If you don't like nature you might like to visit there. I hear the martian rover is ronery these days.
While it may be harmful to put a child at risk of illness without medication, remember in my post which you quoted I said "as long is it isn't harming anyone". In a case where it
is harming someone, I believe it is wrong and should be unlawful.
They that spread this misinformation or fiction as truth also serve the purpose of leading by example. Example of what not to do. You are benefiting from them by learning from their mistakes. As per your example they withhold medication or immunization, you see the outcome and you learn not to make that mistake.
My point is, it isn't always harmful to believe in fiction for a little while. It also isn't permanent. People can think for themselves. They can learn and grow.
The intention of my OP was to squelch the noise from bickering and infighting, anger and vitriol which hinders the dissemination of knowledge.