Need suggestions on 2-3 week trip to NW without reservations

Without reservations, your best bet might be to drive along and check the private campgrounds along the coast. The state parks might be a better/cheaper choice, but most will be full from those who made reservations.

The national forests offer camping without a reservation system, using a "first come - first served" policy. Most of the sites are very primitive. I would assume you'd want something to keep your kids involved, such as a river or the ocean beaches.

Good luck.