I have to agree that Point Defiance zoo is the best! It's like a relaxing version of the regular zoo...there's less to do there, but enough trails for a good walk and the play area will appeal to the kids. However, you have to pay to get in.
If you are looking for free and/or cheaper things to do, don't forget the simpler, the better (simple is probably best for this age as well!) You can go window shopping in downtown Seattle and the carousel might still be up from Christmas. You can feed the ducks at a local park. You can go on a trail walk almost anywhere. You can stay home and make cookies, or make homemade playdough which they will adore. You can go to a local market - or Pike Street Market - and pick out all the goodies for dinner, then go home and make it. You can go to the Seattle waterfront and eat at Ivars and feed the seagulls - there's a carousel and the aquarium on the waterfront too. You can take a ferry ride. You can go to West Seattle for a walk on the beach and lunch out. You can go to a local mall to the play area. You can go out to a McDonalds with a playground. You can go to the library. You can go hunting for Christmas lights at night. You can go ice skating or roller skating if they want to try it. After all that, you will be tired and want to go home for a nap! LOL!
One of my biggest lessons as a parent was when we spent over $100 on tickets to the circus. The kids didn't really seem to enjoy it. Afterwards we walked by the Seattle Center and the giant fountain was on. THIS was the hit of the day...getting wet in the water! For free! They talk about it still (over five years later) and don't remember the circus at all. Of course it was summer, so you probably won't be able to do this in the winter ;)
Enjoy the children, and have fun.