Depends a bit if you want any particular area in Cornwall. You may be interested in River Valley, London Apprentice, St Austell. I am from Cornwall and have stayed there several times. Nice clean site, hard standings, EHU, indoor swimming pool in conservatory style building, childrens play area including a zip wire. About 1 mile into St Austell and also 10 mins from Eden Project. Can,t think of the website but if you search under the name I am sure you will find it.
Also on the banks of a river with cycle hire on site to ride on the river valley trail.
Found Website Cornwall Holidays - Self Catering Cottages, Caravan Park and Camping near St Austell, Cornwall UK