I had them as a child, and it is VERY painful. I remember turning the key every like 3 days, and how much that hurt. I am not saying this to make you fell bad, but to make you aware of what this may be like for your daughter. I suggest lots of milk shakes to make her smile :o)
My 7 year old also needs one, and our dentist and orthodontist have both told us doing it before they are 8-9 is best. Before that age the pallet is still soft, and will spread easier. Also, it will be in the right place when the pallet does harden. That is a biggie. Many people who had braces too young and up having to get them again as adults because their teeth moved back. I am one of those people. In my uneducated opinion (based only on my own experience) I say, get the pallet spreader put in when she is 7.5, and wait until she has most of her permanent teeth to move forward with braces. Even if that makes her 12 or 13. you can get a retainer I am sure to hold the pallet spread in place until then. She would most likely only have to wear it at night. Or, I think they can put a retainer behind her top teeth that she can wear. Just as an FYI, I also had braces on for 6 years because my parents allowed them to start with them when I was too young. I ended up having a few cavities develop under my molar bands because food worked it's way in over the years that brushing couldn't get to.
I totally understand your concern, and share the same questions now that my son is needing the same procedure. I hope I have given you some insight you didn't already have,
T.