My family flew from Minnesota to Australia when I was twelve. I don't really know HOW my parents stayed sane - or maybe they weren't completely lucid to begin with in deciding to attempt it. I'm the oldest of 4 kids. So at the time, we were 12, 10, 7, and 5 years old. And we made the trip back to Minnesota almost exactly one year later. We made it relatively unscathed - tired, but alive, together, AND with all 12 suitcases (plus carry-ons) after 31 hours of travel time.
My family is a group of readers, so we each had several books to help keep us occupied. It was also the first commercial flight experience for most of us, so that made it exciting - at least as far as Hawaii. We flew Minneapolis -> Denver -> LAX -> Honolulu -> Auckland -> Melbourne. One of my brothers made friends with a couple of twenty-something-year-old guys sitting near him and they taught him to play poker - so he was pretty happy from LAX to Auckland. I was sitting next to a couple of older ladies who only talked about designer wardrobes, makeup, and their hairstylists. I got a lot of reading done. On the way home I got almost all the way through "Alaska" by James Michener. My parents must have tag teamed the youngest two pretty well with games and books. I don't remember bringing much for snacks, but that was back when airlines overfed people rather than attempting a slow starvation.
So, I would suggest maybe a couple travel games or small art projects - something new that hasn't been done much at home. I started learning how to embroider and cross stitch around your daughter's age - maybe she'd like to try that. It's a good travel project - doesn't take up a lot of room. Or you could even read aloud some books that they might both enjoy.