Fun onesies are a must. Or fleece pyjamas, but onesies are great as you can go back to your tent in the evening and they put on their pjs or thermals and then throw the onesie over the top before it gets too cold and they are ready for bed.
Crocs or wellies for the morning as the grass will always be wet in the morning even if it hasn't rained. If you don't have camping chairs then a waterproof picnic rug or a tarp for the floor to sit on outside the tent.
Facepaint is good to take. Something that lights up or glow sticks to put on her in the dark so you can see her easily, or a high Vis vest.
You definitely don't want to eat at vans for every meal, it will be extortionate. We take things like pot noodles, pot porridge, cup a soups, Idahoan packet mash with tinned stewed steak or corned beef. When you boil the kettle for hot drinks in the morning, fill an insulated flask for later and you can make noodles/hot chocolate etc while you are in the arena without having to go back to your tent.
Definitely pack marshmallows, kebab sticks and chocolate digestives, there is usually a communal fire at festivals you can toast them at. A cheaper treat than crepes from a van every night.
A picnic blanket for sitting to watch bands, or those foldable travel seats with the back rest are great.
Marvel powdered milk is great for hot drinks or mixing with water for cereal, doesn't have added sugar. Take lots of cereal bars, apples or other fruit that doesn't bruise easily, crisps, crackers, oatcakes, pain au chocolate, brioche etc
Fun clothes/novelty hats, tutus, paper and pens (we use the IKEA kids art cases). Head torch is very handy, especially for portaloos at night.
Baby wipes are handy, but my top tip is take a bar of soap and a sponge or flannel and a mini nail brush in a ziplock bag. Often festival toilets just have hand sanitizer and their hands will get absolutely filthy at a dusty festival. Don't underestimate how nice it is to be able to wash your hands properly after using festival toilets. Home Bargains/B&M usually sell soap filled sponges which are great for this. There are usually taps dotted around and water bottle filling stations, so I just give their hands a good wash when we pass those.