Depends on your style of camping.
I like sites which are quite 'wild' and pitch where you like rather than rows of tents on specific pitches. There is generally a lot more space and it feels more outdoorsy. I choose one where camp fires are allowed and either hire a fire pit or take my folding one.
Pitchup has good filters for finding the style of site that you like.
However this is harder if you want EHU (electric hook up)
Think through your day:
Warm and comfy sleeping conditions - lots of us like SIMs (self inflating mats) which are insulated from the ground cold, but more expensive than air beds.
Cooking: usually a gas stove, one ring with small cannisters, or a two burner stove and a gas bottle? Maybe something like a Cadac Party Grill that you would also use at home for BBQs?
Portable BBQ?
Some kind of table and a chair each. IME kids fall off those little folding stools.
Plates, bowls, cutlery. Pound shops are your friend or just use old / unbreakable stuff from home. Pans suitable for the cooking you plan. Stove top kettle? Unless you have EHU. But you can just use a pan. Matches, tin opener, corkscrew, bottle opener
Washing up bowl , detergent, tea towels.
Lanterns and torches. Spare batteries.
Easy on-off shoes - best to keep outdoor shoes out of the tent especially when wet. Flip flops for showers. S hooks / butchers hooks for extra hanging in showers - put your stuff in a plastic bag.
Wipes. Loads. Anti-bac and for hands.
Coolbox?
Food: loads of good 'pouch food' that doesn't need to be kept cold - microwave rice (just heat in pan with a spoon of water), Canned chilli, etc. Bags of brioche and a satsuma or banana make a good breakfast.
A kite, boules, etc - NO BUBBLES. Campers hate bubbles, the detergent destroys the waterproofing on canvas tents and the 'beading' on poly tents.
Power bank for charging phones or do it in car when you are out and about.