I think at 10 she will be fine, if someone explains to her what to expect. Lie flat, and keep as still as possible. Someone will push the bed into the tube, and give you a button to press if you aren't OK with what is happening. Noisy and may sound a bit clunky but nothing to worry about. Can bring her own CD or have radio on. Someone watching from the other side of the radioactive screen thingy. And if the nurses/docs need to speak to her they will do so by stopping the music and asking her to do what they need, eg "keep really still for the next 10secs," or "just a couple minutes longer" type thing.
For me, when I had one, I felt reassured by knowing that if I wanted to I could stop it at any time by just pressing the button. Knowing they would be able to hear me was a large part of my being relaxed in it.
About half way through mine i realised I was not thinking about it at all, instead wondering the most random things, like how the cleaner managed to clean the inside of the tube. Does someone lay on their back on the bed, get pushed in by a colleague and then swish and swipe with a hospital grade wipe?
For my 10yr old DC, I would suggest bringing an audio book, esp if it's familair as that may give comfort.
I would also focus on being strong for dd and reassuring her that the staff do it all the time and know exactly what they are doing. And reminding her "wont it be great once its over then we will know..." If she is interested, you could go down the route of how amazing it is that drs can see what is happening inside our bodies with this amazing machine, how clever!
I hope the results are what you need them to be, and your DD recovers quickly.