It appears that the scheduled equipment (though the OP did not disclose date of travel) is an ex-NW 757.
Seat selection should obviously be based on personal preference...in this case the deciding factor should probably be bulkhead v. non bulkhead.
On this equipment, the OP's parents may appreciate the ease of the bulkhead entrance and exit, since there will be no passenger seated in front of them.
The galley area will be on the far side of the wall in front of their seats, and should not present too much of a problem.
Also, since the flight is even numbered, they should have first choice on meal selection,
Finally, they will have very easy access to the crew, should they require any special attention. Hope they have a great flight and trip!