mock crocks! I know they are the shoe style forgot but they really come into their own for camping and you can buy kiddy sizes in the £1 shops nowadays.
So handy for slipping on in order to nip to the loo with little people at 5 am, also great for wearing in the shower if the hygiene standards are less than you'd like etc. I'd never go camping without a pair for everyone in the party.
A picnic blanket with a waterproof underside - so many uses as the grass in the UK is always damp even in a heatwave. A total necessity with kids.
My preference is always to take a gilly or kelly kettle in order to guarantee my am coffee supply no matter what! Originally designed to ensure Cornish and Irish fishermen still got their cuppa in the fiercest storm they can handle anything the average camping hol can throw at em.
They don't require gas canisters and can be fueled with moss/twigs/your rubbish so incur no additional expense. I like the idea of an open fire but find my trusty gillie kettle a much safer option with young children/dogs as the the fire is enclosed, (which also means they never fall foul of the fashion for banning open fires some campsites embrace).
Don't get too stressed about food prep would be my biggest tip. I've yet to have a camping trip where we didn't give in to the urge for fish and chips or a local greasy spoon cafe brekkie at least once during our break, (usually after a day the beach or when we wake up to find it's raining cats and dogs) It is after all supposed to be a HOLIDAY!
Most of us can live on a diet of good bread, cheese or peanut butter, fruit (and wine ) for a few days without coming to any great harm. I only ever fret about taking a very,very few items specific to my son's special diet as he is dairy intolerant - otherwise in the UK you are never more than a brisk walk from a shop, or a short drive to a mainstream supermarket.
Online supermarkets will deliver to campsites if you want - a good tip is to get a bread/fruit delivery for the morning after you arrive to save lugging it with you to the site
Do pack a halfway decent small first aid kit including calpol and your favourite remedy for bug bites/stings.