Sound like you're some great grandparents! I'd recommend you fly into Seatac and get a
hotel in Seattle for the night. Then spend nice evening/morning at various seattle attractions like the
Pike Place Market.
Go rent your gear at
REI's Rental Dept. they open at 10am. They have the best gear and good prices. Be sure to hold it in advance if you're coming in the summer they will rent out the gear quick in summer. hint: You may want to reserve two pads per person if you're not a regular camper it will be much softer.
If you're looking for camping location that is Washington go to the
Washington Coast and also the
Olympic Mountains. Its about 3-4 hours from Seattle.
Take a ferry across to Bainbridge and drive the rest.
Let me know if you need a map or specific ideas for camping locations.
Dan