I had braces for seven or eight years, different kinds, but all throughout middle school and almost until the end of highschool I've had some kind of metal and plastic in my mouth. This hasn't been too long a time ago, so I still remember the feeling rather well. Especially at the beginning or just after dentist's appointments it used to be really hard. Usually hurt like hell and at the very beginning the insides of my lips used to get very raw and open. I also remember not being able to sleep because of the pain, not to even think of eating.
I might have to add that I wasn't the greatest eater anyway, but the braces really made it a lot worse and when they were really new, I didn't want to eat anything, not even apple sauce or soup. My brother even put my favourite spaghetty and tomato sauce into the blender, but not even that tempted me to eat. Although, I'll never forget this, because that was probably my first moment as a kid when I realised how much my brother actually cared about me.
I think I didn't really eat for roughly a week back then. However, I did drink soy milk and juice and then eased my way back to eating with banana (and probably other fruit) milkshakes. So, maybe, if your daughter likes those, you could always give that a try, because you can get plenty of good calories from such shakes. Other than that I can tell you from my own experience that even as a young kid you won't die if you don't eat for a couple of days, especially not if you drink some of your calories, as your daughter seems to be doing (e.g. through milk).
All the best, be patient with her, because braces can be hell especially if you're very sensitive and/or are kind of nervous about things going on around and inside your mouth.