We like the Children's Museum. It has NOTHING on Chicago's Children's Museum, but it is good. I would much rather hang out at the Pacific Science Center, but it does serve its purpose for small kids. Another children's sort of museum is Kid Quest at Factoria Mall. We love it! It is far more convenient to attend than the Seattle CM. Free parking! In addition, if you attend on Friday from 5-8:30, it is absolutely FREE for the entire family!
Lots of hands on exhibits. They can play in a hut, ride a metro bus, paint, play music, and explore nature.
Be prepared to pay for parking, in addition to the admission fees.