Generalization

In making a generalization, I believe that we can use either "a"(an) or plural forms.
In the following sentences, are both accepted and correct in generalizing things in the sentences?

a) He treats them as if they were a child.
b) He treats them as if they were children.

The first sentence is in subjunctive mood, the article has to agree with the noun: child.

The second one is in declarative mood or subjunctive one -- or not? In this case the verb noun-concord applies

Marius Hancu wrote:

Only b) is correct.

I agree with Marius that 'He treats them as if they were children' is correct.