This is a tricky one, as most festivals have so much gorgeous food, you could eat at a different stall for every meal and never get through them all. My very favourites are Pieminister, which do the most beautiful pie and mash and the Goan fish curry stall-heaven! I also loved the W.I. tea and cake stall at Camp Bestival last year.
However, this can firstly end up very expensive and secondly, if the food is too exotic it's not always easy finding something that little ones will eat. The main things that were successful with the children last year (then aged 1 and 6) were
Pizza Express Margherita and doughballs
Baked potato with beans and cheese
Sandwiches and cake from the W.I. stall
Sausage in a roll from a post sausage company
My 6 year old also tried the noodles and the pie with mash, but was a big uncertain.
So not really an amazingly varied diet, but enough to survive on for a few days. Our strategy was to take them to a Little Chef for a big breakfast on the first day- this was a posh Heston one with birdsong in the loos, woohoo! This sorted out one cooked meal and then I also try and cook a meal and freeze it, which means it is ready for reheating the next day. Chilli is easy as it can be served with wraps, curry with naan bread, rattatouille with crusty bread. Basically you want to try and cook with one pot wherever possible.
Some years we have the campervan, which gives us more options for food storage, but we dont have electric hook up or gas, so the principle is the same if you are camping, which we will be doing this year.
In your coolbox, freeze as much as you can and this will act as an icepack for your fresh items and obviously lasts a bit longer until it needs to be thrown away. So take frozen bottles of water,milk, frozen bottles of cocktails, frozen sausages, bacon, bread etc etc.
For my youngest, I try and take a high protein breakfast cereal, such as plum porridge, which i think has quinoa in it. Top with fruit puree, which is available from the baby section.
Take plenty of fruit that is portable and not likely to get bashed in a bag if you are out for the day- apples, nectarines, sultanas etc. Leave the bananas at home! Also plenty of healthy energy releasing snacks, such as flapjacks, rice cakes, bread sticks, cereal bars, so the kids are not just eating sugary stuff all day. You can get long life cartons of smoothies, juice and I find the little bottles of strawberry milkshake are great, as they dont need to be kept in the fridge. It is very easy for the kids and adults to get dehydrated, as you are likely to be doing lots of walking/ chasing toddlers/ dancing/ sitting in the sun (ha ha- fat chance!), so I find that taking a couple of bottles of concentrated squash and refillable drinks bottles is useful. You can also buy fruit and veg juice (desperately trying to find a way to get some veggies into the kids) and a big box of cherry tomatoes is good to munch on.
Wraps are lightweight and long lasting, these are good with tuna and sweetcorn.
I know it's not very eco friendly, but I found that I could buy a whole stack of instant coffee/ hot chocolate vending machine cups from places like Home Bargains. If you take a flask of hot water with you, you can save a fortune and keep warm. Take marshmallows for the kids hot chocolate.
You can usually buy staples such as bread, eggs and milk at shops on site and fruit at the farmer's market. But dont forget staples such as butter, oil, tomato ketchup and pot noodles if you are really desperate! Quick cook pasta is also good and could be topped with tuna/ frozen grated cheese/ ratatouille etc.
If your children like them, soya desserts are good as they are high in protein and dont need to be kept in the fridge, same for soya milk which can be used on cereal. You can also buy fruit in jelly which is nice and individuals portions of pineapple in juice were also a hit.