Toms Field… Langton Matravers near Swanage. One of my favourites. No frills but will take campervans with hook-ups in one field and its a short drive to Lulworth.
Toilets/showers/little shop – right on the coastal path, lovely walk to dancing ledge, Winspit and The Sqaure and Compass at Worth Matravers. Even better stagger back in the dark – guess it depends on the 'family' aspect of the trip.