I've driven between Seattle and northern/southern California so many times I really can't count them; a number of those trips have been with little kids, so I understand your point.

Here's my suggested itinerary - http://goo.gl/maps/mdU2X
In lieu of an amusement park (there really aren't any good ones in the Pacific NW) I'd recommend spending two nights in the Columbia Gorge area, based in Hood River, Oregon. Travel there on the first day with a stop at the Johnston Ridge observatory at Mt. St. Helens, a very impressive view over the volcanic crater left behind the 1980 eruption of the volcano.

Hood River is the center of a marvelous area for visits with kids. The Columbia Gorge is incredibly scenic, with vistas, numerous waterfalls, deep forest trails... but also plenty of things for kids and their parents to enjoy. For example, you can visit Herman the Sturgeon, a huge and ancient fish (maybe 80 years old?) at the Bonneville hatchery near Bonneville Dam on the Columbia. I remember visiting that hatchery when I was a little kid, and kids for generations now have loved visiting Herman, seeing the millions of young salmon being reared in the hatchery... great.

But an hour up the (gorgeous) Hood River Valley and you're at Mt. Hood, which in the summer is (IMO) equally impressive, and much more easily accessible, than Mt. Rainier. Visit Timberline Lodge, a beautiful old depression-era structure, and - maybe - there might still be some snow nearby to play in. (There's summer skiing at Mt. Hood but this is a weird winter so no promises.) In either case, a whole day in and around Hood River and the Gorge will be VERY worthwhile.

Columbia Gorge -



Mt. Hood from Hood River Valley -



The next day is a longish but not particularly boring drive down through the Willamette Valley and out to the coast on Oregon Hwy 38, which passes through a lovely river canyon before opening up into a meadowland area (just before the town of Reedsport) where there are usually numerous elk grazing in the pasture off the south side of the road. You then continue south on US 101 through Coos Bay to the lovely little town of Bandon for the night. Bandon's "old town" is picturesque and has a number of quite good seafood restaurants, fishing boats, etc.

The next day is a stunner - along the southern Oregon coast (the most scenic part of the coastline) and into the redwoods in northern California. There are countless pull-offs, beaches to explore, places in the redwoods to stop and gawk at these incredible trees... it's a full and terrific day. Spend the night in Trinidad or Arcata, CA, and if the timing works out, take your son to the Samoa Cookhouse for a family-style dinner - very memorable.

The next day, take the "Avenue of the Giants" byway (parallels US 101 south of Eureka) for some of the best giant redwood groves, in particular the Founders Grove) and then on to San Francisco.

This drive offers amazing diversity - volcanoes, alpine lodges, giant rivers and waterfalls, rocky beaches, the (awesome is the only word) redwoods... all of it terrific for adults and their kids. Highly, highly recommended.