Check out the overview on this page, which lists some excellent kid-friendly activities in Portland: http://vacation.away.com/oregon/travel-v2-cid1844-portland-cid350262-family-vacations.html. You should also schedule some time to get out of the city to go see Mount St. Helens, the Columbia River Gorge (try a jet-boat tour), or Mount Hood. Lastly, I bet your group would love a whitewater-rafting trip down the Clackamas River (http://gorptravel.away.com/gorptravel/rafting-guide/oregon/clackamas-river.html), only about an hour from Portland.