|Ice Cream is also good for keeping cool - according to DD|
So I shall be planning for the worst (ie taking wellies etc) but also expecting and planning for the best!
Take sunscreen. Take enough for the whole family, for lots of flesh on display, for the whole 4 or 5 days of the festival. You will be able to buy it at a festival shop (assuming they don't run out) but it will be expensive and the range will be limited.
Take some sort of cooling device. Take a tent style sunshade for example for babies, a fan, a water spray bottle, an umbrella or sun shade to carry, maybe sun hats (you can often include them as part of your fancy dress)
Dress accordingly, take some loose long sleeved stuff in case it really is baking, some fields have little natural shade. The kids field at Camp Bestival has lots of natural shade in the form of trees, and also a few tents. Check the maps on your own festival plan.
Don't forget the leafy cool of the Dingly Dell (at Camp Bestival) Home to Project Wild Thing this year. A great place to visit at the middle of the day.
The inside of tents can get hot. Carry water bottles (you can refill them at points around the festival) some squash manufacturers now make single servings of squash to add to water bottles, this can be a good idea to get kids to drink plenty.
Remember that however hot it gets in the daytime, once the sun goes down it gets chilly, take some jumpers or blankets either pop back to the tent at teatime, or carry them - the shmangles are good here, doubling as a picnic and lounging mat in the day.
Oh and did I say? Take Sunscreen....