Should our descendants want to know about the beleaguered group of individuals collectively pegged as Gen Z, they could do worse than unearth “Dear David,” the new film from director John McPhail. As a teacher who is regularly in contact with “Zoomers,” I found Mr. McPhail’s unironic depiction of their social media-addled, gender-generous, attention-deficited, multi-culti ways true to form. Kind of adorable, too. Mr. McPhail’s take on these kids makes one a tad less cynical about the future.