I volunteer in the lost property tent at Leeds Festival, so my tips are based upon what I've seen....
Make sure your DCs have key contact numbers written down rather than just on their phones- you would be amazed how many don't know their parents numbers, or even house phone number, and they may need this if they lose their phone.
A good tip is to change your lock screen for the duration of the festival to a photo showing 'this phone belongs to...', a contact number for a friend at the festival and contact email. We have chargers in Lost Property so can charge up phones and call these numbers to reunite phone with owner.
If you are taking meds, label them with your contact number - scary how many inhalers and other meds are handed in, with no way of reuniting them with their owners!
If your DC has a top of the range iPhone and you happen to have an older model that they can use for the weekend - do it. Tents do get broken in to, and showers seem to be a target for thieves regrettably.
At Leeds there's a pretty well stocked Co-op on site. It's not cheap, but food looks ok.
Hope your DC has an amazing time! It's been such a rough year, especially for young people.