Bedding - sheet for bed and top sheet to put blankets on top.
Has caravan got CD player - take some CD's
Take small thing of washing up liquid, tea towels, towels, sugar, salt, pepper etc - toilet roll, kitchen roll etc
I always take a mat to put outside shower as bathroom floor can get quite wet and slippery.
Food - i always take few tins of stuff - baked beans, spaghetti, noodles, pasta and sauce in case nothing suitable in shop on first day.
Remember if you arrive on a Sunday that most of the fuel stations are shut! Dont get caught without petrol.
For the kids - i always take few books, few toys, football etc - swimming stuff, sandals that easy to get on and off as you have to take them off at edge of swimming pools. Beach towels. Calpol just in case
PErhaps some washing powder if you want to wash some stuff through in one of the washing machines.
We take cooler box in car and fill with sandwiches and snacks for journey and then we have cooler box for picnics etc
Which site are you going to?