Don't bring duvets loads of people do and normally it's fine but they spread and touch the sides. If it rains hard and the sides of the tent are touched, the rain comes straight through, and a duvet takes ages to dry.
Do try to borrow stuff if you can it can costs hundreds to buy everything.
Torches and laods pf spare batteires. Camping lantern or battery powered fairy lifts to light up tent. Sleeping bags stay wrapped up and are water resistant on the outside, so even if the tent leaks they're normally OK.
You'll need a cook box for food that needs to be cold. Some frozen meat for the 2nd pr 3rd day (you might still need to defrost so be careful). Meat for first day, dry food stuff, milk, and stuff that's easy to snack on (some good suggestions above).
A camping stove and fuel. Tent, inflatable roll mat or air bed depending on tent height. Camping Chairs, camping kettle, saucepans or frying pan to cook on. A flat surface, either a table or at least a cheap tray, to chop food on. Trays have lips to stop the food rolling away when chopping so better than a chopping board. Something to do the washing up in (some sites have a sink you can use but check). Washing up liquid and tea towel. Clothes and washing stuff for you and everyone, go for stuff that can be layered it cma be freezing at night. I have worn multiple tops and two jumpers before. Blankets for extra warmth and to sit on or put toys on for kids to play with. Entertainment (books and travel games etc) and possibly a battery-powered charger for phones depending on how often you need to charge yours.
Dry food and instant coffee/tea bags.
Spare toilet roll if your using the toilets on site, although you might prefer a porta Potty at night. And if you drink booze and lots of it.
I also suggest asking the other parents what they would bring and what they end up doing.