julie4travel - if they have specifically asked for tops & denim, that's what they want as if they are anything like NC they are trying to have some uniformity on the site. Do they take full length photos? NC only do head shots, so it wouldn't have mattered if DD had a dress or trousers, the only specification we had was bright colours, no pastels.
(I was v cheeky and bought 4 plain t-shirts in H & M, so I could ask them which they wanted on her, keep that one and took the others back next day!!) They did in fact keep her in the Minnie Mouse t-shirt she went in as they were only doing head shots and the Minnie logo didn't show.
It wouldn't do any harm to take a dress with you if you have the room as you'd kick yourself if you saw someone else's dd in a dress when you've forced yours into trousers. If she doesn't want to wear trousers, why not tell her they are her "work clothes" (or whatever bribe you think she'll go for) and that she can put her dress back on after?