Recommended locations 3-4 hours from Seattle with kids?

Ocean City RV Resort
Ocean Mist
The Dunes Beach Resort
Ocean City State Park
Oceana RV & Camping Resort
Quinault RV & Marina

Ocean Shores, WA has all the touristy junk that kids like - go carts, ice cream, pizza, restaurants of all kinds, movie theater, bowling, pool hall, bumper boats, arcades, mini golf, kite flying, Audubon Society, Ocean Shores Interpretive Center, horseback riding on the beach. yadda, yadda, yadda.

Top Ten Things to Do at Ocean Shores
http://www.tourismoceanshores.com/Things+to+Do+in+Ocean+Shores,+WA/PageID/2/default.aspx

For the adults: Quinault Beach Resort & Casino. The casino has a gravel RV parking area for boondocking. No services other than a trash dumpster and shuttle bus to the casino.

If you're thinking about going to a theme park in Idaho, we got the Great Wolf Lodge off of I-5 exit 88 in WA. :B