Just been camping and used self-inflatable mats to sleep on. They were far,far more comfortable than inflatables and easier to put up. We got ours from Aldi for £9.99.
Take sleeping bags and a duvet for warmth and cosiness.
I try to take a couple of meals made at home and frozen. If you get a cool box you can plug in to the car it should keep them cool. It's really nice to have a home cooked chilli, or stew to eat with minimal effort.
Also take lots of baby wipes. They come in handy for all sorts of things.
Have a lovely time!