A few other notes on your trip in terms of kids. Visit Jackson Hole for a few hours heading to the Tetons. With little kids two nights is just right for the Tetons. If you want hookups goto Colter Bay RV--the kids will love it there and it is right next to the marina where you can rent canoes and kayaks for great views and wildlife. Take the boat across Jenny Lake and do a short hike.
In Yellowstone, we split our five day stay between Grant and Canyon to minimize driving. A few tips. There are two places to swim in Yellowstone and both are warm and great. One is the Firehole River just downstream from Old Faithful--great swimming hole for little kids. And the other is the Boiling River just inside the North Entrance--amazing. I also recommend taking the toad on a day's drive through the Lamar Valley and over Beartooth Pass.
The drive to Glacier is easy mostly on 287. Wiith kids we like St Mary's on the East side (reservable and near town but beautiful and great spots for big rigs. Apgar and Fish Creek are the choice on the west side. Your kids will love Glacier and so will you. Many Glacier has great hikes with kids as does St Mary's Lake.
Hwy 2 is a beautiful drive all the way to Sandpoint. Great town on a beautiful lake with lots of camping nearby. Just south of there is a great treat for the kids after camping. Silverwood is an amusement park with great rides and coasters, but is also a fabulous water park. There is a dusty campground there, but camp instead 10 miles away at Farragut State Park, a wonderful place.
Paul is riight that WA 20 is a great choice, but you really can't go wrong traveling across the state. Even I-90 has its charms. You do need to decide if you want to skip Seattle. If you do take 20 you can follow it all the way to Deception Pass--a world class state park. From there you can take a ferry from Keystone to the Olympic Peninsula with great trails, beaches, historic forts, rain forests, and alpine hkes all close. Great camping options there.
You can go around the Olympic Peninsula and rejoin I-5 at Olympia. I would skip the Washington Coast south of the park--it is unremarkable compared to Oregon..You will have to decide for yourself if you want to detour to Mt Rainier. It is well worth it. Mt St Helens is very close to I-5 and there is great camping at Seaquest SP.
Once again, if you skip Portland, you have a great option to avoid traffic by taking US 30 west out of Longview to Astoria, a cool and historic town with lots of great things to do with kids. Awesome camping at Fort Stevens State Park and the KOA across the road is pretty nice too. Heading south on 101 is Cannon Beach RV, a great park with a really nice indoor pool.
Then you are faced with two great choices. Drive the entire Oregon Coast, a fabulous undertaking, or cut in at Reedsport to drive to Crater Lake. Great camping at Diamond Lake just north of Crater.
I am happy to provide details on any place along your trip. We have camped it all over the years. BTW--if you havent been to Lassen NP, consider it. Cool geothemal trails and it is only 5 miles RT to the top of 10,500 ft Lassen Peak on a great trail. Our kids did it at that age with bribery and encouragement and loved it. Lassen is easy to see in a day or two.
Best ot luck
