First things first:
πολύς-πολλή-πολύ is an adjective referring to an amount of something. It is followed by a noun.
exp. Έχει πολλή ζέστη σήμερα.
Ήρθε πολύς κόσμος.
Κάνει πολύ κρύο.
Not to be confused with the adverb πολύ, which is followed by an adjective. (usually. As you can see, the last exampleis different. Πολύ is not followed by anything.)
exp. Αυτό είναι πολύ περίπλοκο.
Η άσκηση είναι πολύ δύσκολη.
Μου αρέσεις πολύ.
Μεγάλος-μεγάλη-μεγάλο is an adjective referring to size. So obviously, it's not interchanngeable with πολύς-πολλή-πολύ. At least not in all cases.
So, τα παιδιά είναι πολύ would be wrong. If you wanted to refer to the actual amount of kids, you'd use the plural form "τα παιδιά είναι πολλά" (plural of the adj. πολύ). If not, you'd have to use πολύ with an adjective, so that it makes sense. Exp. Τα παιδιά είναι πολύ άτακτα - The kids are very naughty. ^.^
Does that mean you disagree with Dimitra's post above? Because she leads off saying that "πολύς-πολλή-πολύ is an adjective referring to an amount of something." But the fact that "numerous" refers to an amount is the exact reason you're giving for why it shouldn't be accepted here.
Or perhaps I'm misunderstanding one or both of you.
The problem here is that this sentence is wrong. Ο πληθυσμός είναι πολύς is wrong, because population is something that's talked about in terms of size, for example large, small. Πολύς refers to the ammount of something. So both πολύς and numerous, which translates to πολυάριθμος, are wrong. ^.^