I've given this a lot of thought - and I have looked at your other posts to get a better idea of your overall trip.
Or course, there are three factors that, combined, limit your options - -
1) Short daily mileage 2) Avoiding routes with significant climbs 3) Using rail trails as much as possible
Most state transportation departments have traffic counts maps or date online. (AADT - Average Annual Daily Traffic)
Here is Washington's: http://www.wsdot.wa.gov/mapsdata/tools/traffictrends/
If you notice, WA 28 has very low traffic. WA 26 also has pretty low traffic counts, too.
Both would involve negotiating the I-90 Columbia River bridge at Vantage.
If you were to do WA 28 - I would suggest cutting up to Cheney - college town with services -
Then cutting over to Edwall and Harrington - and later cutting southwest to George (busier stretch).
If you were to do WA 26 - I would head southeast to Oakdale and Colfax -
With an option to loop down to Kahlotus and Connell for more services - then Othello and Royal City.
Via WA 28 you have the choice of hitching across the river from the boat ramp on the Old Vantage Highway.
That road has a spectacular descent down Frenchman Coulee - with basic camping at the river.
You could call the Vantage Resort and see if you could hire a person to come from the marina.
Otherwise, you would need to get on I-90 and then use extreme caution on the bridge.
Or you could call the resort and arrange a van/pickup shuttle across the bridge for either route.
I have hitched across a number of major rivers in the U.S. - Mississippi, Missouri, Potomac.
If the boat ramp is fairly busy, you just hang out and ask folks.
I was solo - with three it would be a little tougher.
It usually takes no longer than an hour or two at a well-used ramp.
Yet another option is one I outlined before -
Take the WA 26 option via Colfax and Connell then via Vernita bridge and Yakima.
The stretch from Connell to Yakima is remote, but does have minimal services at spaced intervals.
Then you have the gorgeous Yakima Canyon ride up to Ellensburg.
I would also like to mention that not all rail trails are created equal.
It's been years, but riding on the Milwaukee Road route in Montana was brutal - just railbed.
And getting to the start or from the endpoints of some rail trails sometimes defeats the purpose.
Rail trails are often built for local use - not for through riders.
The Trail of the Cooeur d'Alenes is an example.
The eastern section from Lookout Pass to Kellogg is often smack up against I-90.
It was built using environmental remediation funding because of past mining in the region.
Plus to get to the eastern end, you have to do a lot of riding on I-90 in Montana.
A little further north is a spectacular route that connects with the C d'A at Enaville.
From Thompson Falls, you cross the river on the restored High Bridge and then take MT 471 - almost no traffic.
At the Idaho border it becomes paved FR 9 (Forest Road) - Thompson Pass is only like 150 ft higher than Lookout.
Murray and Pritchard have basic services.