You don't indicate what route you intend to take. If you're talking about road bikes on the highway 101 route, I wouldn't take kids that age on it. It's a highway that's great for experienced adult riders, but that kind of riding with kids is taking a chance. You can't at times drift in the lane or, like most highway riding, be anything but super steady--cars and trucks go by 60-70 mph, etc. And around Lake Cresent on the north part of the Olympic Peninsula, there is a 10 mile section, very curvy, with no shoulder. You need to be a savvy, mature, esperienced rider to do this kind of riding.
That said, I admire the idea of getting the kids out there on adventures. I suggest the Trail of the Couer d' Alene, 150+ miles of paved trail )round trip) in gorgeous mountain/forest country across N Idaho--with easy RR grades and no car traffic! Google it and check it out.
Good luck, and have fun.