Parents, please don't worry about feeding your mid on I-Day. Despite your fears, he/she will not starve that day or any other day. There is plenty of food available. Yeah, your mid will probably lose weight during plebe summer due to the workouts and stress during meals. But, he/she will NOT starve and, once Ac Year hits, I guarantee, he/she can gain back weight if so desired.
Worry about making sure you are sufficiently hydrated. There is food and drink available in Dahlgren and the Midstore. In addition, there's the family picnic and also plenty of time to wander out in town.
Your mid will have had so many new experiences in the ~12 hrs from the time he/she starts I-Day until he/she sees you again . . . food is probably the last thing on his/her mind. You could bring some water or a soda and maybe a small snack. But, if you don't . . . your mid is heading to dinner. He/she will get food and drink.
Remember, some mids have no family at USNA on I-Day (my roommate came alone), and they won't be allowed to starve. I know you want to be supportive, but it's really your presence that's comforting, not the food.