Portland for 5 days?
Among those five days, you have time to do the Columbia River Gorge. You will want to bypass Interstate 84 from Troutdale to Multnomah Falls on the historic Columbia River Highway; don't miss it. Now, if you are going that way, consider crossing the Columbia River at the Bridge of the Gods in Cascade Locks, OR (do some research ahead of time and find out why it is called the Bridge of the Gods: your children will love the story), and head just a couple of miles east of the north end of the bridge, visit Skamania Lodge at the west end of Stevenson, WA. There you can eat: http://www.skamania.com/cascade-room.php But read on.
Timing is important: the best time to eat at Skamania Lodge is the time for the Sunday Brunch. Your children will love it as much as you do.
That means that the best way might be to start early Sunday morning and cross the Columbia River on Interstate 5 in Portland, then head east on Washington State Route 14, a scenic and pleasant drive in its own right on a two-lane highway, to Skamania Lodge. Have brunch there, and then cross the Bridge of the Gods southbound back to Oregon and pick up the Columbia River Highway at the east end, wending your way back to Portland past fantastic waterfalls.
Definitely visit the market!
Kenny and Zuke's ( for breakfast or lunch is a good place for Jewish-style deli - great housemade pastrami, baking (bagels, bialys, their own rye, hamburger buns, desserts, etc.) is done in house too. http://www.kennyandzukes.com/
Superdog is a great place for hot dogs and sausages of all kinds...even Zweigle white hots (from the Rochester NY area, and totally delicious). Two locations, one downtown, one near PSU - http://www.superdogpdx.com/
Simpatica Dining Hall does an excellent Sunday brunch and has by reservation only dinners on the weekends that are great: http://www.simpaticacatering.com/
Toro Bravo is a bit away from downtown but worth the drive. Get there at 4:45pm (doors open at 5) and you won't have to wait for an hour or so as they do not take reservations. http://www.torobravopdx.com/
For more upscale dining, Davis Street Tavern is a great new place near downtown: http://www.davisstreettavern.com/
For great pizza and a great caesar salad, try Apizza Scholls: http://www.apizzascholls.com/
A very good meal with nice wines and a family friendly space is Nostrana (have the butterscotch budino for dessert!): http://nostrana.com/
For a fun churrascaria experience, try Brazil Grill: http://www.brazilgrillrestaurant.com/