We self cater every year, pretty much every holiday, and while I now refuse to stay in Dead-Grannies'-Cottage (you know the one - with the 1970s carpet, the oven that hardly works and the damp smell) what is provided varies widely. IF it is UK based it is worth checking trip advisor and website for an idea of what you will get. Or just email and ask owner. We are staying in a great cottage at the moment and it provides everything - loo rolls, cling film, tin foil, dishwasher tabs, wash powder, oil and herbs, bubble bath and so on. This isn't usual.
I'm going to assume UK and suggest:
Take enough food for meal that night and breakfast the next day, so if you are delayed en route then it won't matter if local shops are closed.
If you can book supermarket delivery do so. Or take it with you when you go, you don't want to be shopping when on holiday. Book a supermarket delivery for when you get home too, as you'll be knackered from travelling, have nothing in the house and again won't want to go shopping.
Kitchen: dishwasher tabs, sharp knife, peeler, corkscrew. Small bottle of olive oil, twist of herbs in cling film, twist of gravy powder in cling film. Nick sachets of salt, pepper and sugar next time you are in a cafe to pack. Flask, plastic cups for kids, plastic plates and bowls, box for putting sandwiches in for picnic lunches. Wash powder to keep on top of laundry.
Kids stuff: couple of plug socket protectors if younger, multi plug if kids are older and they have electronic devices to plug in, stuff for beach, colouring books and pens / sticker books. If DC are younger take a bed pad for putting under sheet in case of accidents. I take a small bag with a few "surprises" in for wet days / slow mornings. Usually lego friends animals bling bag things, a new DVD, the sticker books.
Randomly: map of area, binoculars, guide books. Any tech you can't live without (we have taken Chromecast for the last two holidays). My coffee machine which I need to be able to function as a human. Wine.
I'm totally of the "everything and the kitchen sink" school of packing. Boot is stacked. But we have fun. Hope you do too.