I am a near novice and may be mistaken, but it seems to me
(1) This is a very good question.
(2) None of the responses tackle why it would read "pueri pluralis est", using a singular verb and singular adjective to describe a plural noun.
Question: Are singularis and pluralis third declension adjectives, as I am guessing? The following assumes they are.
I am using Jenney's First Year Latin, 1990 edition. On page 50, introducing adjectives, I am told,
"If one adjective modifies nouns in different genders, it is masculine plural if the nouns refer to persons; neuter plural if to things."
As puer, puella, and pueri all refer to persons, could it be that there is an implicit "he" (Latin "is") as subject with which the verb and adjective are in agreement? Then, in the case that two nouns referring to persons were spoken of in the same sentence, the subject would shift to implicit masculine "they" (Latin "ei"). Like so,
"pueri pluralis est" == "(he) is plural"
"puella singularis est" == "(he) is singular"
"pueri et puellae plurales sunt" == "(they) are plural"
Whereas if nouns referring to things were spoken of, the subject would be neuter, singular "it" (Latin ""id") or plural "they" (I can't find the Latin for neuter "they"), depending on whether the subject were singular or compound. Like so,
"terra singulare est" == "(it) is singular"
"terrae plurale est" == "(it) is plural"
"timor et terra singulares sunt" == "(they) are singular"
Just a thought. Right or wrong, you gotta love this stuff. It makes you think!