With kids, Villefranche-sur-Mer works.
Many people's favourite spot on the whole coast: a perfect sheltered bay of impossibly clear water, a tumble of ochre houses, a tiny sand-and-shingle beach, and a citadel — all a 10-minute train ride away. There's little to 'do' beyond swim, eat and wander, which is precisely the point; that's also why it's a half-day, not a full one.