We had McDonalds chocolate milkshake over Sound of Music White Sailor dresses one time. Thankfully I was looking after the boys, so we smugly stayed out of the way while the panicked washing went on. None of the gilrs would even admit to who brought it in. 
Chocolate is standardly banned backstage when I've been there. I tend to do more musicals, so we tell them chocolate is bad for their voices (apparently true) and they're usually pretty good about not having it, again it tends to be parents who object.
99% of the children are delightful. They're excited, but nicely so.
You get the odd one who thinks they know it all and why should they do as you say (like minding out of the way for someone else's entrance, had an 11yo refuse point blank last year). That's when you need to have back up from the ballet teacher. They've only got to once tell them that if they do it again then they won't go on, and they're usually obedient albeit sulkily after that.
I have a lovely certificate my last group gave me "For being the best chaperone"-I have it up on my wall. Really touched me, I knew they were up to something, but I thought it was a "pinch something from the boys" type game, but a couple of them had done it, and left it in the toilets for them all to sign, so they then all asked to go one after the other.