Subject + に + 形容詞/形容動詞

Question

I understand that に can be used to mark the agent of an intransitive verb, but I often see it being used with adjectives instead. For example, in the sentence:

私にはできる
As for me, I can do it.

に here marks the agent of the verb. The same thing can be said about 私にはわかる. However, there are cases where に are followed by adjectives, which, although I intuitively understand the meaning, do not understand the grammar behind them.

私には金が必要です。
As for me, I need money.

寝るには早すぎる。
It's too early to sleep.

電車に詳しいです。
to know a lot about trains.

体にいいです。
It's good for your body.

子供に怖い×
Scary to children. (apparently incorrect.)

What is the function of に（は） here? In what situations can this be used?

Edit: I found a related question, and this linked one, but the first one seems to have conflicts about in which situations you can do this and the second question only uses に to mark the agent of intransitive/potential verbs. This question also asks something very similar, but the answer doesn't really address why 「子供に怖い」would be wrong.

Answer 1

Ok, I asked a native about this, and this is what I got from the conversation. As @naruto says, I seem to have put a lot of different usages of に into the same generalization.

体にいいです。
It's good for your body.

子供に怖い×
Scary to children.

These seem to be contractions of にとって. I couldn't find a reason for why 子供に怖い sounds unnatural to most speakers, but this contraction seems to only work for some adjectives. I was also told that 子供に怖い映画 sounds slightly more natural because there is a noun attached to the end.

寝るには早すぎる。
It's too early to sleep.

Verb+には seems to be a fixed pattern. IMABI seems to say this can be translated to "In verb + ing". So a more literal translation would be "In sleeping, it is too early."

私には金が必要です。
As for me, I need money.

This is the purpose marker に. It shows who "needs money" and can only be used with certain adjectives like 必要.

電車に詳しいです。
to know a lot about trains.

This is the contraction of の場合には as described in this post. So, "In the case of Trains, (I) know a lot about them". If I got anything wrong, please correct me. I am not 100% sure that this is correct. Also, if you are willing to give a more detailed (or accurate response), please do.