Christie Brinkley is happy to pose in a bikini at 59 but not for every magazine. The model, who looks great in a swimsuit in Social Life, was taken aback when a guest at a Water Mill bash asked if she’d be willing to pose for Playboy. A spy reports, “Christie said, ‘No, because it would embarrass my children.’ ” Brinkley was at the bash with her brood, including Alexa Ray, Sailor and Jack. Also there: Social Life’s Devorah Rose, Consuelo Vanderbilt Costin and Jean Shafiroff.