naif, as defined by Vocabulary.com
noun: a naive or inexperienced person
.....naif is [similar to] to the adjective naive. They share a common
origin, the French word naïf, which means both "natural, unspoiled, or
innocent" and also "foolish." When you describe someone using the
adjective form of naif — which can be used interchangeably with naive
— you are usually implying that the person is a little childlike or
adjective: marked by or showing unaffected simplicity and lack of
guile or worldly experience
When I first saw the question, I immediately thought naif. Then I saw it in comments, and assumed somebody was busy writing an answer. But no, maybe because Colin Fine (see comment) doubts that 2% of English speakers would recognize the word "naïf". Assuming he is right, that would be about 20,000,000 people worldwide. IMO, enough to justify an answer!
As for a collection of naïfs, I suggest "a nursery of naïfs".
Example Sentence (made up and plausible for any political commentator writing for a literate audience):
Only economic naïfs believe that X's policies will
do anything to help the middle class.