Vashon Island, WA. It's a really badass place, 15 minute ferry from Seattle. I used to go every summer with my parents. I loved being there as a kid I'm sure yours will too, there's lots of interesting shit on the shore (i.e. geoducks that spray you with water and giant starfish). There are a lot of really nice camping areas as well. Check out the lighthouse and the beach at the Isthmus.