First, the question is out of left field. The ungrammaticality of
- *Any children didn't come
doesn't have a thing to do with subjects. It has to do with how one uses the word any,
which is rather a complex subject.
There are at least two English words any:
Free-Choice any, as in Pick a number -- any number, or Anybody can do that.
Free-choice any can be a subject, but it usually occurs with a modal like can or able.
It's a special term with a special grammar and meaning, basically "Choose one".
Negative Polarity any. This is by far the most common use of any, and it (and all its compounds like anybody, anyone, anywhere, etc.) is a Negative Polarity Item. That means it can only be used grammatically inside the scope of a semantic negative trigger. These include negatives, questions, and a lot of idiomatic constructions.
Negative polarity items are determined by removing negative triggers and seeing whether the NPI is still grammatical. E.g, the boldface NPIs below:
- I haven't ever been there. ~ *I have/I've ever been there
- She hasn't been here in weeks ~ *She has/She's been here in weeks
- He doesn't have any assistants. ~ *He has any assistants
- This shouldn't take long. ~ *This should take long
Questions are negative environments, as it happens, so NPIs can occur there,
- Have you ever been there?
- Does he have any assistants?
- Will this take long?
even as subjects.
So, since NPIs have to occur with negatives, the reason why *Any children didn't come is ungrammatical is because any isn't in the scope of a negative trigger. Not any children would be clunky but grammatical, though No children is better. There's also upstairs negation, as in
- It's not true that any children came.
- I don't think that any children came.
- He didn't claim that any children came.
where the negative in the main clause licenses the any in the complement clause.
tl;dr -- If a sentence sounds odd, see if there's a negative in it, or whether adding negation makes it better.