Out in the rain was coming down on the children.
The sentence doesn't make sense, does it? It was written by one of my students several years ago. Perhaps the writer started with a sentence idea something like this:
Out in the rain the children were getting wet.
In the process of writing, the student perhaps decided to recast the sentence into something like this:
Outside, the rain was coming down on the children.
Instead of making a complete change from one structure to another, the writer mixed the two structures in one sentence.
The problem with the mixed construction that we saw first is that one word, "rain," is forced to perform two roles. It is both an object and a subject, and that's why the sentence doesn't work.
"Out in the rain" is a prepositional phrase. "Rain" is the object of the preposition. "Rain" is also the subject of the sentence, and one word cannot be both a subject and an object.