One of the minor, pleasant surprises of light rail is what a great amusement ride it is for small children. I’ve used it extensively for that purpose, and I can tell you that most little kids can easily become totally obsessed with the line and its stops.
One problem is that there isn’t a ton to do for kids within little-feet walking distance in many station areas. That’s certainly true when the weather is poor. Fortunately, Delicious Baby has a two-part series (1,2) to lay out lots of outing ideas for each station.
The only item I’d add at Seatac is going up to the top floor of the parking garage to watch the planes take off. If you’re OK with taking a connecting bus (or walking a longer distance) that widens the options to some pretty great parks in the Rainier Valley (Jefferson, Seward, and Kubota Garden).