Prepare for all weathers: wellies, raincoats, hoodies, ponchos and suncream. You can never be too prepared.
Lightweight fleeces are good for when it starts to cool down in the evening and you might want to tuck in a blankie or two for when kiddies get tired, they can just lie down and cuddle in while you enjoy the music.
If space allows try pack some cards, miniature board games or even activity books. Or bring tablets/iPad. They will come in handy in the evenings, waiting in queues or if you just want to chill out in the afternoon.