Hey QS,
I don't know where you're located but the University of Washington's School of Fisheries has a hatchery right on campus.
You can watch chinook & coho ascend the fish ladder and be gathered for spawning right in Seattle on Mondays, Wednesday & Friday mornings.
Then, there's the Cedar River at the south end of Lake Washington and the Wallace Hatchery just up river from Sultan on the Skykomish to name a few.
If you want a real treat, head up to the Weaver Creek Spawning Channel on the Frasier just over the border in British Columbia.
Lots of options and I'm sure you won't have much of a problem getting the kids a salmon spawning "eye full"!
By the way, I think that's a wonderful thing you're doing for your children!!!