K.B.
I agree with the responders below, zoo-lights at Pt. Defiance (tacoma) is fun. The beach is right there too so the kids could do that as well. If you go to the zoo, save your tickets that give you money off to NW Trek and make a day to see that as well. I tend to like the childen's museum at Seattle Center more than the one in Tacoma. There is a lot more for the kids to do there. the Pacific Science Center is also at Seattle Center.
Also on Thursday's in the newspaper there is a section of things going on in the area over the weekend, they are broken out into categories and can be really helpful in finding things to do.
Hope you have a smooth move up here and welcome to the area!