Welcome to Flyertalk.
Given the details you've shared -- you've only flown one time, three kids who are first-time fliers with various diagnoses, trip to Disney on the line -- I'm going to strongly suggest you find a non-stop flight, even if it means flying into LAX and taking a car service/shuttle/Uber or paying slightly more.

By booking a connecting flight, you're doubling your chances that something goes wrong. I understand that you want five assigned seats close to you, you want to carry on your luggage and you're understandably concerned about delays. One thing you need to understand about flying is that you have to be prepared for the worst-case scenario. So let's say you buy the connecting flights through SEA. What happens if you do get delayed and miss your connection? If one leg gets canceled? If there is an equipment change and even with booked seats you're suddenly reassigned in five seats spread across the cabin? Are you planning to buy trip insurance? Do you have flexibility in your schedule if your return gets delayed? To be clear, any of these could happen on a non-stop flight, too, but they're twice as likely to occur if you're taking a connecting flight.

To more directly answer your questions and comments:
* Call Delta and ask what the "minimum connection time" is for Seattle. I believe it's 30 minutes, but the reservations agent can confirm. If you book on a single ticket, that the minimum time you can have between flights.
* 5-hour tarmac delays: I don't fly to Seattle regularly, but I find this hard to believe. If you search for the specific flight number on FlightStats.com, you can see a flight's on-time performance. (It is calculated from gate to gate, not wheels up to wheels down, so tarmac delays would factor into on-time performance.) The 5 am Spokane to Seattle flight, for example, was on time 75% of the time in the last month, and the average delay was 40 minutes.
* Southwest: There are a few ways to qualify for early boarding. If you have disabilities or need extra time, if you pay for early boarding, or if you're a family. Presuming you can split into two groups (1 adult/1 child and 1 adult/2 children) I can promise you that in any of the three early boarding situations you outline above, you can get 3 seats together and 2 seats together.