I had to do this for my daughter (a few years ago) when in Alexandra. I was lucky. Just a few for about 20/30minutes. My experience was that a) most of them were too excited and too much in front of the mirror to eat!, and b) they wouldn't touch anything messy.
Toothpicks were my saviour on the evening. Anything that can be taken in the mouth from the end of a toothpick should go down well! Nothing wrong with cheese,pineapple,gherkin etc. Pieces of fruit interspersed. Marinaded or crumbed "bites" of fish or meat (purchased or cooked yourself) they can "spear" and dip into a sauce.
Carrot sticks, celery sticks,pieces of capsicum, rice crackers etc. with a firmish dip or two.
Of course, you'll also be catering for males and parents. I am merely suggesting a few nibbles for the young ladies!
Good luck, I treasure the photo of my eldest daughter on the evening of her first ball.