When do "Premium" seats open for assignment?
 

My daughters are traveling without me... When do Preferred seats become available for assignment? Can you snag them online or at airport only?

You are GM and should be able to request the elite medallion line to allocate seats with you or close to you.
Have traveled with my children and even though they are on separate PNRs ... have been able to get seats with mine/ close to my seat.

If you call in ... in the worst case they will say 'No'.
My experience has been the CS reps have been very helpful.

It is very unlikely that you will find an agent who will place them in a premium seat if you are not traveling with them. 2 hours before departure you can put them in a premium seat though. You can do this online or at the airport, but IIRC, the iPhone app *does not* allow you to switch into premium seats within the 2 hour window, even though the website does. If you have already checked in you will need to see an agent or use a kiosk (?) to change your seat.