I think there are some real misery guts on this thread. Talk of sucking up, and one-up-manship. As a cub leader, I think this sounds lovely, if a parent thought to send a box of chocolates or fancy biscuits on a cub camp, we'd be made up, and the would very much be enjoyed around the campfire in the evening once the kids are in bed. Even cheap chocolates! It's not so much what it is (although chocs are much appreciated), it's the thought behind it - that a parent has acknowledged how hard work it is to take away other people's kids, and has at least tried to do something that might help, even in a very small way.
We're lucky if a parent/child remembers to say thank you when they pick up their child from camp. We are usually dead on our feet - we've had little to no sleep for 48 hours, have put up umpteen tents, shelters and marquees, made sure all the kids are ok and are entertained, dealt with bad behaviour, dealt with homesickness, occasionally actual sickness, wet beds etc, cooked, cleaned, washed up, organised and ran activities, done first aid and medicines, cleaned toilets, and still have to take down all the tents, pack up the trailer, unload at the other end and put everything away after the cubs go home. Some parents are appreciative and will say thanks, but many won't remember to. Some will stop to help pack up if we ask, but most won't. So any little gesture that says thanks - before or after the event - is lovely.
Just say thanks when they get back and move on. It's their job. Teachers don't need more food, stuff etc etc. It's not a teacher's job to take kids away on a residential, it's voluntary, they don't get paid any extra, and they are on duty 27/7. It's hard, unpaid work.