Takeaways, or very easy meals. Maybe get pizza bases and sauce, grated cheese, and let them make their own pizzas with a range of toppings to choose from? (Doesn't need loads, a couple of slices of ham, some pepperoni, 1 mushroom finely sliced does a large pizza, sweet pepper, onion, pineapple, …. whatever they usually like - and mostly any leftovers can be eaten as snacks while it cooks or used again another time).
Bring snacks, and organise easy snacks for when you get back from outings/swimming etc to keep them from getting feral. Carrot and pepper sticks, bread sticks, chunks of cheese, crisps, bite-sized pieces of fruit, and something to drink. Different nibbly-bits every day, but prepared in advance or bought while out to pull out once you get back to the van. (A few Ziploc bags can be very handy for pre-preparing snacks or ingredients for meals!).
Bring a deck of cards and a favourite or new board game. And a couple of favourite DVDs or download some movies to an ipad from Netflix etc before you go. And a football or frisbie.
Extra socks!! And a warm jumper/hoodie each for any cooler evenings. And waterproofs if you have them, so weather doesn't matter nearly so much.
Ignore dirt - persuade them to swim frequently, wash occasionally, and embrace the freedom of outdoor life.
If it does rain, pull on waterproofs and go out anyway - jump in puddles, splash in the sea, explore rock pools, notice different wildlife in woods in the rain, play football or frisbie, …..
Tell everyone to bring a reading book, for quiet time when you are needing to cook/organise dinner etc. Or for older ones to read during naptime for younger one(s). (And if either of the older 2 need a nap, encourage it - you can do things differently on holidays, and there might be a night they want to stay up later/they might wake earlier with light/birds so they might need some extra sleep another time/day).
Give the older ones some pocket money to buy a trinket/icecream/sweets etc for themselves.
Also, give the older ones some jobs - like sweeping the floor while you organise something, or tidying the outside furniture, or setting the table for meals - make them more responsible because "Daddy (is so sad he) can't come so we all need to help so we can all have fun".