Pringles, Pringles and more Pringles. And other snacks. Really, kids that age chow down snacks.
When my son and his friends were that age, a bucket and shovel went a long way toward keeping them occupied. Just make sure they fill in their creations to avoid camp hazards.
On a Grand trip earlier this year, Spikeball kept the "adults" entertained. This is the best beach game I have seen in a while. Endless fun and spirited competition for all ages.