Congratulations on your expanding family!

I know NW 86 arrives in Seattle around 9 am, but after commuting every weekend between SEA and the East Coast, I think the latest red-eye flight from SEA is 1 am. Usually in the summer there is a 12:50 am and a 1:00 am! (I've seen people board the wrong one many times.) There is usually a 3:00 pm, 4:00 pm and sometimes a 6:00 pm departure from SEA to MSP.

So, I think you're looking at a 9:30 am arrival and a 2:00 pm departure. Given immigration, customs and baggage recheck at SEA S Terminal, even if you arrive at SEA early, you most likely will not leave the S Terminal for the main terminal via the train until 10:15 am at the earliest. (This does not give you a lot of time to exit, leave the airport and return for a 2:00 pm flight.) Then again, the SEA WC (which closes at 2:00 pm) is not the best place to be stranded for several hours, either.

You should note when you depart the airport how long the security line (even the Elite/FC line) is for TSA. In the summer, even the "fast" line can take well over 30 minutes midday.

Good luck, and keep looking for decently-priced alternatives involving fewer segments.