I've tried the light rail to and from Sea-Tac and total trip time is much longer than you'd think. So to answer the OP's question, yes, you will be wasting a lot of time.

The light rail line wanders serpentine-style through various neighborhoods south of the city, criss-crossing I-5 about three times. Scheduled duration from Westlake Center (heart of downtown) to Tukwila (nearest stop to Sea-Tac) is 34 minutes, but add waiting time for the train, waiting time for the airport bus shuttle, and bus shuttle transit time, and the one-way trip can take well over an hour. The trains dispatch on seven-to-ten minute headways.

(I left my downtown office at 800am, bought my train ticket at 811a, arrived at Sea-Tac at 915a... and barely made my 945a flight -- had to beg to jump the security queue and run to the gate as the GAs were closing it out.) Meanwhile, the 194 bus takes about 30 minutes, and you can drive / taxi from downtown to Sea-Tac in 20 minutes or less.

So... bottom line... the light rail, many years and billions of dollars in the making, is stupid and I won't use it again. Sadly I believe Metro is retiring the 194 bus when the light rail line is finally extended the last two miles to the airport, forcing people to use the slower and more expensive rail option.

On both my light rail rides there were only five to ten people on the whole train. The subsidized cost per passenger must be astronomical -- enough to put each of us in a chauffeured town car. Such is the determination of Seattle city planners to force rail transit, no matter how little sense it makes. (I'm a clean-bus man myself.)

The still-not-done airport station is being built several hundred yards from the terminal -- passengers will have to hike a long distance through the chilly, drafty, lonely parking garage -- even though numberless cities around the world, from ORD to CLE to SYD to LHR to even PDX, manage to bring rail transit directly to the terminal, usually underneath it.

Finally, to the OP: if you intend to ride the light rail back to the airport at night, I would try to pick a train / car with a security officer aboard. Some of the neighborhoods where the trains stop en route are a little sketchy, and the stations are open-access at street level. I wouldn't send my family on that route after dark, myself.