One year we collected five dollars from each kid and bought them breakfast each morning of the test. One day we had pancakes on a stick from Sonic, the next we ate chicken biscuits from Chick-Fil-A, and the last morning we ate breakfast tortilla (or something like that) from McDonald's. We also served juice each morning. The kids loved it! We had a number of kids who couldn't afford to bring in five dollars so the principal covered their costs. Other years we've just had peanut butter crackers, donuts, muffins, of something similar.
I find the kids are worked up enough and that it's best to keep things simple so that they will be calm for the test. I have given them things such as a granola bar or a cheese stick and a piece of fruit. These things are simple to pass out, simple to eat and simple to clean up. That's what I would look for...you and the kids will have enough to do that you won't want to have used your mental energy up on putting together a big breakfast. Keep in mind that a lot of kids are too nervous to eat so a simple snack maybe all you can get them to nibble on before a test. Good luck!!