[PHXFlyer]

Your post is not completely clear. Did you cancel the tickets because of the unaccompanied minor fee or because something changed with the travel plans?

In any case, you must accept that the the tickets were both non-refundable AND non-transferable. AA and NW are two different companies. Just because AA made an exception and issued the vouchers for the girls' tickets in your name doesn't mean that NW automatically must or will. You can keep sending e-mails but you've already received NW's response several times and they're not likely to budge on the matter.

Regarding the UM rules and fees, all airlines are indeed different. I'm sure your 14 year old granddaughters are perfectly capable of traveling alone. Not all 14 year olds are. The airlines are simply "covering their butts" with a blanket policy that if a child hasn't reached their 15th birthday, they can't travel alone unless declared an unaccompanied minor. I think they chose this age with the FAA exit row restriction in mind. A 15 year old can occupy an exit row seat. I guess the airlines figure if they're old enough to sit in the exit row they're old enough to travel alone.