Huge hugs hun. I can hear your anxiety though your post.
I haven't ever had to go through anything like this with my kids, so I don't know in reality, but just being removed from the situation and the anxiety, I would say,

Yes to an overnight bag, JIC. Maybe even some clothes for you/DH too.
Yes to nappies, again, JIC. She might be in a little pain and might not want to get up to go to the loo for the day?
Yes to entertainment, something quiet, like her favourite books, maybe movies or game on an i-phone/portable DVD player or something like that, textas/paper ect...quiet activities she can do in a hospital, and while maybe she might be in recovery.
Yes to the nurofen. The doctor will probably tell you what is best to give her, but if she is in pain and really needs it, you can give her both panadol and nurofen at the same time, they act differently and have different active ingredients in them, so will help her with her pain more effectively.

Maybe bring some snacky foods/packaged drinks ect, for you and DH, but maybe for her too. Take something to keep you busy, movies on your phone/portable DVD, books, magazines, puzzles...I know you might not be able to concerntrate, but best try to keep busy while she's in surgery.