I've been a longtime lurker, this is my first post, but I have to write it because I can identify with your daughter. If you were to ask me, it's not the wholegrain roll or the cheese....it's the salmon. i was sent to school with salmon sandwiches and curried egg sandwiches among others (the smell was just delightful after a few hours in my lunchbox
). All I wanted was to fit in with everyone else eating vegemite sandwiches. I saw a devon sandwich with tomato sauce as a treat!
Never fear though, by the time I reached high school, my 'fancy' sandwiches were coveted by other friends, I even had to make extra so that I could share with my friends who had 'boring vegemite sandwiches' (that I didn't want to eat, hence making more of mine)
Perhaps make a plainer style sandwich, ham instead of salmon or something, and see if that makes it easier for her.