I've seen a few ideas for making turkeys out of fruits and veggies and other healthy foods... www.eatingwithfoodallergies.com/turkeyvegg
or Cornucopia fruit salads using waffle cones www.tammymitchellphotography.com/2011/08/1
I also think if you get the kids involved with helping to prepare the food, they're more interested in eating it. They can do stuff like helping to stir things, older kids can peel potatoes, stuff like that. It might be a bit hard with 25 kids but maybe you could work out "kitchen shifts" or something.