On the first link, Axioplaseさん explains:
体にいい is 体に(対して)いい, while 子供に怖い is 子供に(とって)怖い. It's a completely different grammatical form, with a different "subject". In one case, the subject is the-thing-that-is-defined-by-the-context-but-isn't-in-the-examples, and in the last case, the subject is the kids.
According to him that's why「体にいい」 is correct and 「子供に怖い」 isn't.
Still according to Axioplaseさん:
These vegetables are good for the body.
When you are a kid, this movie scares you. (Not "this movie is scary for kids", that's the point).
I think the は after 子供 is important. If you don't have it, then 怖い looses its subject, and your sentence becomes wrong.
That last paragraph is about why one can't rephrase 「子供にはその映画が怖い。」 to be 「その映画は子供に怖い」.
On the other hand, on the second link broccoli forestさん explains that:
is grammatical only because 必要だ can take two arguments (AがBに) as word-specific feature, not that it's a universal grammar for adjectives. It's like the English word worth can have an object but most other adjectives can't.
If I'm not mistaken, 「僕にはペンが必要です。」 could also be written as 「僕にとってはペンが必要です」.
So why wouldn't 「子供にその映画が怖い」 be a correct variation of 「子供にとってその映画が怖い」, meaning "this movie is scary for kids"?
My questions are:
- How do I know an adjective can be used with the 「に」 particle at the same time as 「が」? In which case would the word marked by 「に」 be the subject?
- Is it okay to abbreviate 「に対して」 and 「にとって」 to 「に」? Is that what's happening whenever 「に」 is used with adjectives?
- Why does 「子供にはその映画が怖い」 mean "when you are a kid, this movie scares you" and not "this movie is scary for kids"? Why is 「その映画は子供に怖い」 unacceptable?
P.S. I relied entirely on other questions and even mentioned other users. I don't think this is forbidden, but it may be frowned upon. I hope I was able to be clear enough so that no one needs to click on the questions I linked to understand this one. And I apologize if I shouldn't be bringing up other questions or other users so much like this. I also apologize if I misunderstood any of the mentioned people's answers.