There are many small bays in Jersey with the name Petit Port or Petit Portelet. This one is easily accessible, so no trailhead waypoints! Parking is available on the nearby roadside. The beach offers a great spot for a picnic, especially after visiting Mont Orgeuil Castle, and is usually very quiet.
You do NOT need to climb over any fences. If you visit at night, you will see the castle floodlit.