Grammar rule: ONE sentence; ONE subject, ONE predicate. Is it?

I just watched a video on grammar (https://www.youtube.com/watch?v=Drv6jD8xWdw) that states that English sentences can only have one subject. At first, I thought it was obvious, but then I thought of some exceptions.

"I found it strange being in her house"

"It is wonderfully beautiful, the chair"

"They’re very polite, your children."

Then I realized that they did actually have just one subject, even when the pronoun reiterates it. But then I came across a way more interesting example:

"Those kids work very hard, most of them".

Structurally, it is the same as "They’re very polite, your children", for example. But in this case, "most of them" is not an equivalent of "They". It is just a portion of the subject. So, I guess that it is to assume that "most of them" is the actual subject and "those kids" something different.

I am aware this sort of sentences tend to omit the pronoun, as in: Very polite, your children. But you can’t really do that in this case without changing the whole order of the sentence.

What do you think? It really got me thinking. Does this have two subjects?

PS: Some other member suggests that this other post could solve the problem. I don’t think it does, but I find it interesting and complementary. Omitting "is", like in "I think it strange"