Just organising our school camp at the moment and have a meeting with parents this week about it.
Can I suggest:
Twice as many pairs of socks as you think they'll need.
Make sure there are two pairs of shoes/trainers in case one gets wet. If they aren't able to do up laces easily, then send the ones with the velcro straps.
Extra hair ties. I always take a few packs and end up giving them all away because the parents forget to send spares. Make sure your child can put their hair up in to at least a ponytail on their own.
Don't send jeans, just tracksuit pants or leggings. Quicker to dry.
For any toiletries you send, then make sure they are able to open the packets - none of the child proof lids. Put things in small travel containers - no big shampoos (they won't use it anyway!). Don't send a big cake of soap, it just gets left in the shower or ends up making a mess in the bag.
Don't send any item of clothing that you aren't prepared to lose, destroy, get muddy or wet. No name brands, it only causes a headache when Mummy bails up the teacher upon return wanting to know why the special name brand jacket has mud on it.
Unofficially, make sure there are some snacks in the bag. There will always be a "secret" midnight feast that the teachers know nothing about. 
Don't send any toys (other than a bedtime cuddle toy), trading cards, legos, balls, electronics, barbies, shopkins or any other item. There are activities planned all day and evening, there is no time to play with them, we already have a full set of sports gear with us, and it just means more to carry (and lose).
Do NOT send your child with their bloody Furby and expect the teachers to keep the damned thing alive. The next idiot that does this will find their stupid creature fed to the penguins. And I'm not even kidding.
Make sure that whatever bag you send with them that they can carry it themselves. Teachers aren't bell-boys. We're taking 70 kids with us to camp, and it's not our job to carry all the bags that are too heavy to be lifted, because the parents over packed. One bag per child.
Optional extra: The parent who came to pick up their child from the bus and handed me a bottle of wine is still my favourite parent ever.