Parents send everything from sandwiches (meat, chicken, avocado, etc.) to left overs, which is easily heated in the microwave. They send cut up fruit and steamed vegetables, cut up cheese with crackers. It seems to work well for me and I do not have to do a lot of preparing. Parents are told that they must send nutritious snacks and lunch.
What ever works for the caregiver and makes your day run smoothly is all that is important. I am just glad that I do not have to cook for 7 kids everyday.

Cheers