Everyone seems to have a list of "must haves" for festivals with children so I thought I would share mine. We've been to about 10 festivals with kids now and have definitely found somethings that work and others that just sit untouched the whole time.

Obviously, we always bring a trolley :)

This year I invested in wool trousers for my boys. They are great under waterproofs during the day and under pajamas at night. They stay dry pretty well and my boys seem warm enough in them, even on cold nights. I also bring a wool sweater for each of them. I pick a sweater that is large enough to accommodate as many layers at the weather calls for.
Wellies are a must have but I found with my youngest (1) he can't walk in wellies. He ends up in his socks within 2 seconds of being put outside, soaking his entire outfit. How in the world can you keep a child warm and dry when their shoes keep falling off? So frustrating. For the solution, I went to my favorite kids shoe store and presented my case. While my boys got busy playing in the toy room (see why its my favorite?), I looked at the options. We got him some waterproof (gortex) boots meant for the winter. They are lined on the inside (so soft!) so I don't need to layer on the socks and they.... stay on his feet! He even did a test run through a giant puddle about 8 times and came out with dry feet. It was amazing. Definitely a good investment. I would recommend for any little tots whose wellies just won't stay on.
Home Baked Treats. I always try and make at least one treat for each festival. It is so nice to pull out a container of homemade flapjacks or muffins for a snack. Though we have been known to eat only flapjacks for an entire day... I also bring lots of snack foods I know the kids like. I also like 5 a day fruit shoots. Once the juice is gone, the bottle can be refilled with water until it disappears, as things tend to do when put into my children's hands.
Lights! Lots of them. Flashlights (aka torches), headlamps, glowsticks, fairy lights, glow in the dark cutlery. If it glows, it is welcome.
As for toys, we only bring 2 toys and 2 books/child. Usually, the toys sit untouched because the kids want to be playing with all the cool stuff going on at the festival or interacting with the wonderful people. They don't want to play with their boring toys from home. We bring a few for any inside play time. I do count glowing cutlery and glow sticks as toys.
Wipes. Pack what you think you will need and then throw a few extra in the car, just in case. You can always go pick them up (or anything else you didn't feel like carrying/need when you arrived) later. In fact, this is a great destination when your children refuse to sleep and you need a break. Put them into your trolley and go for a walk to the car park for more wipes. Less excitement so the kids can fall asleep and a great time to catch up on the day for the adults. I have met so many parents who can't get kids to sleep at festivals, for us, taking them for a walk in the campsite works every time.
That is all I can think of at the moment other then the usual (sleeping stuff, clothes, nappies, toiletries, plastic bags, hats, gloves, scarves, sunglasses,...)

I know I have read some posts about having a to go box to make leaving easier which sounds amazing and I have tried that. It doesn't work for me. I do better just to pack the dishes/cups/etc... that we normally use after doing the dishes and then wash it and put it away when we get home. Also, then I don't have to own multiple sets of plates, cups, etc... which I love.

What are your must haves? Or what have you found you don't need that you thought you would?