The Seattle Children's Museum is really best for little kids, anywhere from walking to about 10 years old. Although called a museum, it really is more of a giant play area. There are kid friendly real-life exhibits (like a grocery store, theatre set, café, etc...) where kids can engage with items and act like grown-ups. There are also a number of activities put on by the staff during the day - so there's really always something going on. We especially like the back exhibit that shows different neighborhoods from different parts of the world. Your kids will most likely be running from one exhibit to the other and they should be pretty tired by the end of their visit.