You should pack an extra towel, dry clothes, snacks, and drinks.
There are concessions items sold at most swim meets but swimmers generally don’t have long enough breaks between events to purchase concession items so it is advisable to pack some snacks.
You should also pack heavy clothes (sweat shirts, sweat pants, etc.) to keep the swimmers warm during the meet.
Some kids bring card games and hand-held games to play when they are not swimming. Just remember that water will ruin most everything!