«Beach Lane» by Melissa de la Cruz

Three girls with three agendas and the ultimate destination: the Hamptons.
Summer in the city? Way overrated. Everybody who's anybody in New York City summers in the Hamptons. Mara, Eliza, and Jacqui all want a piece of the action, all for different reasons.
So the girls answer a classified ad to become au pairs. How bad can it be, watching a couple of kids on the beach all day? They've got the swank address, the sweet ride, and an all-access pass to the hottest social scene on the East Coast. It's shaping up to be the summer of their lives.