You want a ready supply of things to hand out. The obvious ones are, crisps, nut bars, individual soreen bars, made up bags/pots of sweet mix ups etc.
Frozen capri sun and similar keep your stuff cool while camping and are refreshing when its hot.
Make cake tray bakes or flap jack, customise to your kids taste.
Have different coloured pots/bags for each child.
Take small toys for when they just need a bit of time to themselves in the throng.
Wraps. Hot dogs. Bags of rice. Quick cook pasta. Make and freeze sauces.
If you can afford it give them all a budget for the day for ice cream, cake, brightly coloured eye catching stuff etc