What to feed football players between two-a-days

I have three sons, all of whom play football. This week they begin two-a-day practices. They usually don't eat before practice because they will throw up while working out. But during two-a-days, they practice for two and half hours, break for one hour, then practice for two and a half hours more. They are starving between practices but can't eat heavy before the 2nd practice. Any ideas on what to feed them that won't make them sick?