How is the “surfer culture” NOT historical? Surfing goes back centuries in Hawaii, and there was a sort of “golden age” on the west coast in the 1960’s with significant impacts on US culture with TV shows, popular, music, fashion, etc. Sure, we aren’t used to thinking of something within a lot of people’s current lifetimes as “historical” but I think the argument could be made.
Ask your kids what they’d like to be and see if you can do it. They have to wear it after all. It should be something they’d be able to get into character for.
If you’ve got enough kids, you could do all of that, PLUS with a camera and tripod, you could turn it all into a Hollywood movie set.
If you want to represent California, you could dress them up as Mexicans.
I vote for either surfers or forty-niners.
I live in the middle of gold rush country, and all the festivals are going on right now. Girls generally wear pioneer-type clothes, and the miner-type guys just wear jeans and red flannel shirts–the filthier, the better. Jeans were invented here, so it’s accurate. Just give him a shovel and a pan and a very old hat, and you’re golden. Oh, and Alaska had a gold rush too, so you can count that in as well!
Well, it seems to me that Hollywood has really had a worldwide cultural impact. I’m not saying that’s agood thing, but it’s hardly debateable.
Sooooo maybe the girl in a long dress with costume jewelry and an “Oscar”? and the boy in a traditional Hawaian or Alakan native costume. . .
The trouble is that you need more than two. . . you can cover the West coast and Alaska with a Pacific Coast Native American costume and a forty-niner, but then Hawaii is completely out of the picture. Maybe a ship’s captain of the period when Hawaii was first becoming a trade stop instead of the forty-niner? The ship’s captain would have been moving between hawaii, Seattle and Juneau I think . . . or maybe make him a whaler?
One really captivating story to represent is the sled-dog team that carried diptheria serum from Anchorage to Nome 1925 I think? 1925 serum run to Nome - Wikipedia
lumberjacks would work too, jeans, flannel shirts, work boots
I’d go with the prospector look. It’s relatively easy to create. It’s historical. And it covers several states in the west (you can accessorize with a lei if you want to include Hawaii).
Now, can I stretch your generosity and ask if you know any online source for photos of that time (westward settlers, prospectors)?
I don’t know if this is a cynical statement or just a statement of the fact that California was originally part of Mexico (and before that, a colony of Spain)–though the colonial occupation went up to only San Francisco. There’s a reason why the Russian River is called the “Russian River.” (Hint: the Russians were there.)
“Tradition” is one of the hardest things to define in California. (After all, our governor is Austrian.)
Prospectors are generally depicted wearing typical “western” clothing, but carrying a pick-axe and/or a pan for panning gold, and having a pack mule. 
Settler women are usually depicted in long shirt-waist dresses, with bonnets and aprons.
I was going to suggest a T-shirt, jeans, and a baseball cap, but I see I’ve been beaten to it. 
Going as a surfer sounds like an excellent suggestion.
I supposed you should feel lucky the kids weren’t assigned Iraq. From a Robot Chicken sketch: Linus, Lucy, and Charlie Brown from “Peanuts” are shown.
Lucy: I love Geography Week! I got Italy.
Linus: I got France.
Charlie Brown: I got Iraq. (Pronounced like “a rock”)
Total cruise wear. California, Alaska and Hawaii have that in common.
