With a range of dietary requirements, I would stick with each family doing their own catering. It is hard doing big mass dishes on a camping stove, unless you are very well set up for it. Do a couple of more communal BBQs but otherwise cook family by family.
Just think of the simplest meals you do at home, but with shortcuts:
Straight to Wok noodles are brilliant for a stir fry.
Microwave rice pouches can be tipped into a pan with a sprinkling of water and re heated.
Cous cous is quick and easy.
Pasta with a jar sauce and any additions you fancy.
Good non-chilled items include tinned sweet corn, tuna, jars or tins of artichoke hearts, tinned bamboo shoots and water chestnuts for stir fry, tinned cannellini beans to put in a salad. Jars of roast red peppers, olives, sundries toms etc from Lidl or Aldi, good for adding to rice or pasta sauces.
Wraps are versatile: make fajitas, or burritos (tinned re-fried beans, pouch ‘Mexican rice’, tinned chilli), or just make a big ‘quesadilla’: put grated cheese between 2 tortilla wraps, add anything you fancy, tuna, sweet corn, salsa, and quickly dry fry in a frying pan each side. Good for lunch.
Quick cooking veg (courgettes, mushrooms, tomatoes etc) fried in a pan, out in a wrap with slices of fried halloumi.
Tofu to have with stir fry.
Packs of felafels fried or BBQ, in a wrap with hummus and salad.
Be adventurous with your BBQ. Veg kebabs, marinaded chicken, halloumi kebabs, mushrooms if you have a BBQ pan with holes (TKMaxx often sell these), pork steaks etc.
Good extras; packets of brioche rolls and the rolled pancakes with chocolate filling, bags of pain chocolate in individual wrappers, tinned pineapple, squirty cream.