If your kids are obsessed with airplanes, the museum of flight is a wonderful place to visit. For older kids, there are lots of old airplanes to look at (and a few to climb onboard), including the original Air Force One. There is also the chance to tour the original Boeing "factory" and a section dedicated to space travel. For younger kids there is a smallish play area with a few planes to climb-on board and some interactive exhibits that help kids learn about flying. A mock control tower overlooks Boeing field. While there isn't a ton for young kids to do, there is a lot of space, and my kids always seem to enjoy a visit, especially on a rainy day.
I don't often recommend museum gift shops, but the one at the Museum of Flight has a great assortment of airplane and space travel related toys and clothing. I especially love the cute NASA flight jackets and the airplane mobiles.
The Seattle Aquarium is part of the CityPASS joint ticketing system, an Admission Pass to 6 Seattle attractions a savings of 43% at CityPASS. that also lets you skip the ticketing line.