Since you'll already be in Northern California, I'd highly recommend driving the coastal highway up the Oregon Coast! There are so many wonderful views and cute little towns along the way. I've lived near the Oregon Coast most of my life and will never stop enjoying traveling there. Some fun kids locations on the coast (if you don't mind paying some entrance fees) - Trees of Mystery just sound of the Oregon/California border. It's a forest of redwood trees that have grown into various funky shapes. We lived in Southern Oregon when I was 4 and I absolutely loved going there. I also enjoyed The Prehistoric Gardens in Southern Oregon when I was 4. Your kids are a little older, so they may or may not be interested. It's basically a collection of dinosaur statues which I found endlessly fascinating as a dinosaur-obsessed 4 year old. Central Coast has the Sea Lion Cave. It's a giant cave where hundreds of Sea Lions hang out. There's an entrance fee, but I always enjoyed it. You can see Sea Lions other places along the coast for free, but not as many all at once. If you don't want to pay, Newport is just a little farther north and usually has sea lions hanging out in their downtown bay area. Speaking of Newport, there's an aquarium there, but it's pretty expensive and probably not too different from what you could find at home. They also have other touristy attractions - wax works, Ripley's Believe it or Not, etc. I'd personally skip all those and just spend time walking around the beach towns and on the beach. Further north, the Tillamook Cheese Factory is fun and free to tour. Seaside is my favorite beach town, but that's pretty far north. It's got more of a carnival atmosphere with arcades, bumper cards, stuff like that. And as other posters mentioned, the farther north you go, the more likely you are to run into weekend crowds from Portland.
There are tons of options for camping along the coast. I'm not much of a camper so I can't make any recommendations, but many of the state parks have yurts you can rent and they seem quite nice. I've always wanted to stay in one, but you have to plan ahead because they book out fast. I assume you are visiting in July 2017, since you mention next July? If so, you'll want to start looking at campground reservations now. I'm not sure how far in advance they take reservations, but you'll want to find out and then make your reservations as soon as possible. The yurts and cabins are especially popular, and reservations fill up quickly!
You may already know this, but it can be quite cold on the Oregon Coast even in July. Definitely bring plenty of long pants and jackets, including a rain jacket. Even when the days are warm, evenings and mornings can be chilly. Even on a nice day, it can be super windy on the actual beach! Actually, bring stocking hats too, in case you're there on a windy day. Most Oregonians visit the beach wearing jeans and a fleece jacket year round. :)
As for visiting Seattle, I'd only recommend that if you really want to see the San Juan Islands. I've never been to the islands, but they are supposed to be amazing. Otherwise, driving to Seattle probably isn't worth the extra driving for you. Visit Portland instead, it's the same cultural experience. Craft beer, local wine, excellent coffee, amazing food and we have Powell's Books! Powell's would be another fun activity for kids, if they enjoy reading. It's a huge local bookstore that fills a city block. If you don't mind some extra driving you could visit Mt. St. Helens or Mt. Hood, both relatively close to Portland.
