I agree with Dave that if you plan to see any of the sights just north of San Francisco, then Eureka is probably too far to try to get on that same day. Instead, think about taking your days driving north to Seattle and splitting them 50/50 between driving and sight-seeing. That would leave you several hours each day to visit places, while still letting you get about 250 miles (or so, but not much farther) down the road. Sticking to an overall mix like that would make your first stop (after San Francisco) at Willits, then maybe Crescent City after a day exploring the Redwoods, Corvallis after a visit to Crater Lake, and Portland after your drive up the northern Oregon coast. Note that those stops assume you follow the routes I laid out for you. You don't have to, of course, but whatever route you decide to take, keep that 50/50 mix in mind. It's way too easy to think you can do it all, but in the end fewer stops and a less frenetic pace will lead to a more enjoyable experience.

Also note that stopping in larger towns will give you a better range of options for accommodations. Unfortunately, we don't have bed and breakfasts like you may be used to in Europe. Here they tend to be more boutique-style lodging with prices to match. We do have an outfit, called Airbnb where people rent out homes (typically) for relatively short periods of a few days. That might be worth looking into, but for single night stays, they don't work very well.

And finally, don't sell your youngest short on the Junior Ranger Program. English helps but is not a requirement. Mostly the program involves giving the child some activities to perform so that he can learn a little something more about the park. The older children (and adults) can translate for the youngster if needed, but the 'rewards' for completing the tasks include badges and certificates that make great mementos of the trip and I'm sure the park rangers will be glad to make accommodations rather than leave him out in the cold.

AZBuck