We've just got back from a Keycamp holiday in Holland.
We found that with the exchange rate, even using the supermarkets off site, everything was pretty expensive. I'd take salt, pepper, tin foil, clingfilm, sandwich bags, oil, coffee, tea, sugar, oil, washing up liquid, soap, anti-bacterial spray, hand washing liquid if you're going to wash, ketchup, salad dressing etc, cordial, cereal, fun size chocolate treats (depending on your children), other snacks / cereal bars etc for out and about, kebab sticks if you'd use them for BBQ, charcoal, matches. Its obviously nice to have the local fresh produce, but if you can save having to buy things that you'll only use a little of, its worth taking them if you can fit them in.
Equipment wise ? the "mobile home" was surprisingly well equipped. I wouldn't bother with pans but did take a sharp bread knife, a chopping board, a veggie peeler and a corkscrew with bottle opener. Also took scourers, cloths, tea towels, pegs, the children each had a drinks bottle, took Tupperware for picnics, picnic rug. We took our own bedding ? I "double sheeted" ? 2 fitted sheets on each bed because I'm just a bit funny about it ? singles for the children with sleeping bags and pillow cases, doubles for us and our own double duvet and pillows. It did get a bit chilly in the evenings in the caravan so although blankets were provided, if you're wary of how clean they might be, I'd take extra bedding too just in case.