This should probably go without saying and it’s grim but please ask your DCs to NOT pee (or the other) anywhere except in a toilet, even if they have to queue. Aside from polluting the ground, If the weather is dry then it reeks, if it’s wet then they’ll be wading through it. An overriding memory of Leeds/Reading (wet weather years) was groups of lads pissing against a fence at the top of a hill and it running into literal ponds then onward into peoples tents. The fence was next to some admittedly smelly but functional loos so no excuse but they seemed to think it was really funny.
Roads around the event are bad on access and egress. If you can try to collect them outside of the main egress time then you’ll have less of a queue.
Phone signal generally patchy to terrible - tell them to download the set lists and save them to their pictures beforehand as the apps might not work.
As someone else said about the phone Lock Screen with a mates details. Likewise if you’re a parent doing a pick up - best to have one of their mates numbers too in case your DCs phone has no charge/has got wet etc.
Good shoes! Open toed/flips flops/gelly shoes may seem like a great idea in warm weather but they blister/toes will get stood on and are easier to roll an ankle on uneven ground or after a few shandies…
Usual suncream, hat, sunnies , try to drink at least some water, some waterproofs and also something warm - tents can get surprisingly cold and damp at night after you’ve been huddled in a warm crowd all day, warm leggings/top/dry socks to slip into can be a relief.
Take nothing that they can’t afford to lose - unfortunately some people are opportunistic.
Encourage them to take their tent and kit home with them. (It’ll save them spending out again next year if nothing else…)
If they feel unsafe or unwell then the welfare crew are there to help. They’ve literally seen everything. There will be police on site. Again - they’re there to help.
Ear plugs! Not just to help with sleep but also to take the edge off near loud venues - you can get decent ones on Amazon or at boots for not much money at all.
Try not to worry too much - they’ll survive even if some will sleep for three days afterwards and very much decide they never want to camp again!
Top tip for parents picking up DCs if it’s been a wet one: couple of Bin bags on your car seats as they might be a bit soggy and a bin bag in each footwell for them to step into and remove their grubby footwear. Be prepared for a late night McDs or KFC trip… (get the second nearest one into the sat nav before you head off for minimal queueing…).
You can give them all the tools/advice but it’s up to them if/how they go about it. Most of them will have a great time. And for what it’s worth I hope it stays dry this weekend.