A child porn angle on manners and mores among pubescent Manhattan private schoolers, and their families. Never fear, nothing sordid happens: 13-year-old photography buffs Scan Abrahms and Marc Campbell take some photos of Marc's beauteous eight-year-old sister Tiffany, in her bikini (which she happens to be wearing), because Marc wants to convince his father that Tiffany should be a model. (Insofar as the Campbells inhabit a Park Avenue duplex--which is also said, incongruously, to have a view of Central Park--the Brooke Shields analogy is pretty far-fetched.) And when Mr. Campbell has the roll of film developed (taken, contrivedly too, on his Hasselblad), the photo lab alerts the D.A.'s office and the boys, owning-up, are in prospective trouble. All is straightened out at the hearing, however--where along with an airing of child-porn problems (the pix, innocent or not, could have gotten into the wrong hands, etc.), narrator Sean performs so creditably that he wins his carping father's admiration. Fifty pages elapse, in fact, between the photo session and the blow-up-having mainly to do with family peculiarities (a photo-club project has the kids make symbolic portraits) and the infinite varieties of juvenile sexuality (Sean's older, feminist sister baby-sits a macho nine-year-old, given to soul kisses.) Meanwhile Sean, short and diffident around girls, finds his match/complement in shorter, more forward Joanie. In the social anthropology department, he's also bar mitzvahed--on his own, roots-seeking initiative and (at his sister's instigation) by a woman rabbi. As for the child-porn aspect, it's both unobjectionable and extraneous.