I'd say take them to Kennedy Creek west of Olympia off of Hwy 101. They even have guides who walk you up the section of stream, giving information as you go.

http://www.theolympian.com/2012/10/25/2296998/chum-salmon-spawning-to-begin.html

My kids have been on this trail walk a few times. A must see for folks who haven't done it with their kids.