Pauline at the Beach (1983) on Netflix

In the end of the summer vacation, the fifteen year old Pauline travels with her older divorced cousin, Marion, to her seaside house in Granville. While on the beach, Marion meets her old friend Pierre, who has a crush on her, and his new acquaintance Henri, who lives in Oceania and is spending vacation with his young daughter in the resort. In the night, they go to a casino and Marion has one night stand with Henri, feeling a great passion for him. However, Henri is a wolf and has no interest in having any commitment with Marion. Meanwhile Pauline listens and observes the behavior of the older people while she has a little but disappointing romance with a boy of her own age.